diff --git a/lib/pleroma/reverse_proxy.ex b/lib/pleroma/reverse_proxy.ex
index c7ee47c6e..11f041c57 100644
--- a/lib/pleroma/reverse_proxy.ex
+++ b/lib/pleroma/reverse_proxy.ex
@@ -59,7 +59,9 @@ defmodule Pleroma.ReverseProxy do
   * `inline_content_types`:
     * `true` will not alter `content-disposition` (up to the upstream),
     * `false` will add `content-disposition: attachment` to any request,
-    * a list of whitelisted content types
+    * a list of whitelisted content types for which `content-disposition: inline`
+    is always set (overriding any upstream header) so the media can be embedded
+    in pages; the filename is derived from the content type
 
   * `req_headers`, `resp_headers` additional headers.
 
@@ -386,7 +388,27 @@ defmodule Pleroma.ReverseProxy do
 
       List.keystore(headers, "content-disposition", 0, {"content-disposition", disposition})
     else
-      headers
+      if opt == true do
+        headers
+      else
+        name = inline_filename(content_type)
+
+        disposition =
+          if name do
+            "inline; filename=\"#{name}\""
+          else
+            "inline"
+          end
+
+        List.keystore(headers, "content-disposition", 0, {"content-disposition", disposition})
+      end
+    end
+  end
+
+  defp inline_filename(content_type) do
+    case MIME.extensions(content_type) do
+      [ext | _] when ext != "" -> "inline.#{ext}"
+      _ -> nil
     end
   end
 
diff --git a/lib/pleroma/web/activity_pub/object_validators/attachment_type_sniffer.ex b/lib/pleroma/web/activity_pub/object_validators/attachment_type_sniffer.ex
new file mode 100644
index 000000000..19215b627
--- /dev/null
+++ b/lib/pleroma/web/activity_pub/object_validators/attachment_type_sniffer.ex
@@ -0,0 +1,59 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentTypeSniffer do
+  @moduledoc """
+  Best-effort MIME sniffing for remote attachments whose declared type is
+  unusable (missing or `application/octet-stream`).
+
+  Fetches the first chunk of the URL and runs it through libmagic. Only
+  `image/*` results are returned; anything else, or any failure, yields `nil` —
+  so this never reclassifies non-image media and never blocks ingestion. This
+  exists because some remotes (e.g. extensionless Cloudflare Images URLs) ship
+  attachments with no usable `mediaType`, which would otherwise render in the
+  timeline as a clickable link instead of an inline image.
+  """
+
+  alias Pleroma.HTTP
+
+  require Logger
+
+  # libmagic only needs the file header; a few KB is plenty and keeps the
+  # request cheap even when the origin ignores Range.
+  @sniff_bytes 8 * 1024
+  @timeout 5_000
+
+  @spec sniff_image_type(binary() | nil) :: {:ok, binary() | nil}
+  def sniff_image_type(url) when is_binary(url) and url != "" do
+    do_sniff(url)
+  rescue
+    e ->
+      Logger.debug(
+        "Attachment type sniff failed for #{url}: #{Exception.format(:error, e, __STACKTRACE__)}"
+      )
+
+      {:ok, nil}
+  end
+
+  def sniff_image_type(_), do: {:ok, nil}
+
+  defp do_sniff(url) do
+    headers = [{"range", "bytes=0-#{@sniff_bytes - 1}"}]
+    opts = [pool: :media, timeout: @timeout, recv_timeout: @timeout]
+
+    with {:ok, %Tesla.Env{status: status, body: body}} when status in 200..299 <-
+           HTTP.get(url, headers, opts),
+         false <- blank?(body),
+         {:ok, %{mime_type: mime}} <- Majic.perform({:bytes, body}, pool: Pleroma.MajicPool),
+         true <- String.starts_with?(mime, "image/") do
+      {:ok, mime}
+    else
+      _ -> {:ok, nil}
+    end
+  end
+
+  defp blank?(nil), do: true
+  defp blank?(""), do: true
+  defp blank?(_), do: false
+end
diff --git a/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex b/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
index 5ee9e7549..b6e30a438 100644
--- a/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
+++ b/lib/pleroma/web/activity_pub/object_validators/attachment_validator.ex
@@ -6,6 +6,7 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentValidator do
   use Ecto.Schema
 
   alias Pleroma.EctoType.ActivityPub.ObjectValidators
+  alias Pleroma.Web.ActivityPub.ObjectValidators.AttachmentTypeSniffer
 
   import Ecto.Changeset
 
@@ -61,9 +62,27 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentValidator do
   end
 
   def fix_media_type(data) do
-    Map.put_new(data, "mediaType", data["mimeType"] || "application/octet-stream")
+    current = data["mediaType"] || data["mimeType"] || "application/octet-stream"
+
+    resolved =
+      if sniffable?(current) do
+        case AttachmentTypeSniffer.sniff_image_type(data["href"]) do
+          {:ok, mime} when is_binary(mime) -> mime
+          _ -> current
+        end
+      else
+        current
+      end
+
+    Map.put(data, "mediaType", resolved)
   end
 
+  # Only sniff when the declared type carries no real information. Everything
+  # else (including unrecognized-but-present types) is left as the remote sent it.
+  defp sniffable?("application/octet-stream"), do: true
+  defp sniffable?(""), do: true
+  defp sniffable?(_), do: false
+
   defp handle_href(href, mediaType, data) do
     [
       %{
diff --git a/test/pleroma/reverse_proxy_test.exs b/test/pleroma/reverse_proxy_test.exs
index ec4470379..5f75f2a07 100644
--- a/test/pleroma/reverse_proxy_test.exs
+++ b/test/pleroma/reverse_proxy_test.exs
@@ -298,21 +298,21 @@ defmodule Pleroma.ReverseProxyTest do
     end
   end
 
-  defp disposition_headers_mock(headers) do
+  defp disposition_headers_mock(headers, url \\ "/disposition") do
     ClientMock
-    |> expect(:request, fn :get, "/disposition", _, _, _ ->
-      Registry.register(ClientMock, "/disposition", 0)
+    |> expect(:request, fn :get, ^url, _, _, _ ->
+      Registry.register(ClientMock, url, 0)
 
-      {:ok, 200, headers, %{url: "/disposition"}}
+      {:ok, 200, headers, %{url: url}}
     end)
-    |> expect(:stream_body, 2, fn %{url: "/disposition"} = client ->
-      case Registry.lookup(ClientMock, "/disposition") do
+    |> expect(:stream_body, 2, fn %{url: ^url} = client ->
+      case Registry.lookup(ClientMock, url) do
         [{_, 0}] ->
-          Registry.update_value(ClientMock, "/disposition", &(&1 + 1))
+          Registry.update_value(ClientMock, url, &(&1 + 1))
           {:ok, "", client}
 
         [{_, 1}] ->
-          Registry.unregister(ClientMock, "/disposition")
+          Registry.unregister(ClientMock, url)
           :done
       end
     end)
@@ -328,6 +328,40 @@ defmodule Pleroma.ReverseProxyTest do
       conn = ReverseProxy.call(conn, "/disposition")
 
       assert {"content-type", "image/gif"} in conn.resp_headers
+      assert {"content-disposition", "inline; filename=\"inline.gif\""} in conn.resp_headers
+    end
+
+    test "forces inline for inline content types overriding upstream attachment", %{
+      conn: conn
+    } do
+      disposition_headers_mock([
+        {"content-type", "image/png"},
+        {"content-disposition", "attachment; filename=\"filename.png\""},
+        {"content-length", "0"}
+      ])
+
+      conn = ReverseProxy.call(conn, "/disposition")
+
+      [disposition] = Conn.get_resp_header(conn, "content-disposition")
+      assert String.starts_with?(disposition, "inline")
+      refute String.starts_with?(disposition, "attachment")
+    end
+
+    test "forces inline based on content type even without a file extension in the url", %{
+      conn: conn
+    } do
+      disposition_headers_mock(
+        [
+          {"content-type", "image/jpeg"},
+          {"content-length", "0"}
+        ],
+        "/original"
+      )
+
+      conn = ReverseProxy.call(conn, "/original")
+
+      [disposition] = Conn.get_resp_header(conn, "content-disposition")
+      assert disposition == "inline; filename=\"inline.jpg\""
     end
 
     test "with content-disposition header", %{conn: conn} do
@@ -340,6 +374,132 @@ defmodule Pleroma.ReverseProxyTest do
 
       assert {"content-disposition", "attachment; filename=\"filename.jpg\""} in conn.resp_headers
     end
+
+    test "with inline_content_types: true leaves upstream headers untouched", %{
+      conn: conn
+    } do
+      # opt == true: the proxy must not synthesise or rewrite content-disposition.
+      disposition_headers_mock([
+        {"content-type", "image/png"},
+        {"content-disposition", "attachment; filename=\"upstream.png\""},
+        {"content-length", "0"}
+      ])
+
+      conn = ReverseProxy.call(conn, "/disposition", inline_content_types: true)
+
+      assert {"content-disposition", "attachment; filename=\"upstream.png\""} in conn.resp_headers
+    end
+
+    test "with inline_content_types: true does not synthesise inline when upstream is absent", %{
+      conn: conn
+    } do
+      disposition_headers_mock([
+        {"content-type", "image/png"},
+        {"content-length", "0"}
+      ])
+
+      conn = ReverseProxy.call(conn, "/disposition", inline_content_types: true)
+
+      assert Conn.get_resp_header(conn, "content-disposition") == []
+    end
+
+    test "with inline_content_types: false forces attachment for everything", %{
+      conn: conn
+    } do
+      disposition_headers_mock([
+        {"content-type", "image/png"},
+        {"content-disposition", "inline; filename=\"pic.png\""},
+        {"content-length", "0"}
+      ])
+
+      conn = ReverseProxy.call(conn, "/disposition", inline_content_types: false)
+
+      [disposition] = Conn.get_resp_header(conn, "content-disposition")
+      assert String.starts_with?(disposition, "attachment")
+    end
+
+    test "with inline_content_types: false derives attachment filename from the URL basename when no upstream filename", %{
+      conn: conn
+    } do
+      # No content-disposition header: the attachment branch falls back to
+      # attachment_name, which by default comes from MediaProxy.filename/1
+      # (the URL basename).
+      disposition_headers_mock([
+        {"content-type", "image/png"},
+        {"content-length", "0"}
+      ])
+
+      conn = ReverseProxy.call(conn, "/disposition", inline_content_types: false)
+
+      [disposition] = Conn.get_resp_header(conn, "content-disposition")
+      assert disposition == "attachment; filename=\"disposition\""
+    end
+
+    test "with inline_content_types: false honours an explicit attachment_name opt", %{
+      conn: conn
+    } do
+      disposition_headers_mock([
+        {"content-type", "image/png"},
+        {"content-length", "0"}
+      ])
+
+      conn =
+        ReverseProxy.call(conn, "/disposition",
+          inline_content_types: false,
+          attachment_name: "custom.bin"
+        )
+
+      [disposition] = Conn.get_resp_header(conn, "content-disposition")
+      assert disposition == "attachment; filename=\"custom.bin\""
+    end
+
+    test "forces bare inline for a whitelisted type with no MIME extension", %{
+      conn: conn
+    } do
+      # image/x-foo-bar has no entry in MIME's database, so inline_filename/1
+      # returns nil and the disposition should be the bare token "inline".
+      disposition_headers_mock([
+        {"content-type", "image/x-foo-bar"},
+        {"content-length", "0"}
+      ])
+
+      conn = ReverseProxy.call(conn, "/disposition", inline_content_types: ["image/x-foo-bar"])
+
+      [disposition] = Conn.get_resp_header(conn, "content-disposition")
+      assert disposition == "inline"
+    end
+
+    test "honours a custom inline_content_types whitelist", %{conn: conn} do
+      # image/bmp is NOT in the default whitelist; with a custom whitelist
+      # that includes it, the proxy should force inline.
+      disposition_headers_mock([
+        {"content-type", "image/bmp"},
+        {"content-disposition", "attachment; filename=\"upstream.bmp\""},
+        {"content-length", "0"}
+      ])
+
+      conn =
+        ReverseProxy.call(conn, "/disposition", inline_content_types: ["image/bmp"])
+
+      [disposition] = Conn.get_resp_header(conn, "content-disposition")
+      assert String.starts_with?(disposition, "inline")
+      assert String.ends_with?(disposition, "inline.bmp\"")
+    end
+
+    test "treats a type outside the whitelist as attachment even on custom whitelist", %{
+      conn: conn
+    } do
+      disposition_headers_mock([
+        {"content-type", "image/png"},
+        {"content-disposition", "attachment; filename=\"filename.png\""},
+        {"content-length", "0"}
+      ])
+
+      conn =
+        ReverseProxy.call(conn, "/disposition", inline_content_types: ["image/bmp"])
+
+      assert {"content-disposition", "attachment; filename=\"filename.png\""} in conn.resp_headers
+    end
   end
 
   describe "content-type sanitisation" do
diff --git a/test/pleroma/web/activity_pub/object_validators/attachment_type_sniffer_test.exs b/test/pleroma/web/activity_pub/object_validators/attachment_type_sniffer_test.exs
new file mode 100644
index 000000000..52b1b67a1
--- /dev/null
+++ b/test/pleroma/web/activity_pub/object_validators/attachment_type_sniffer_test.exs
@@ -0,0 +1,178 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentTypeSnifferTest do
+  use Pleroma.DataCase, async: true
+
+  alias Pleroma.Web.ActivityPub.ObjectValidators.AttachmentTypeSniffer
+
+  # @sniff_bytes must mirror the value in the module under test; we assert the
+  # Range header that the sniffer sends, so the test should fail loudly if the
+  # constant drifts.
+  @sniff_bytes 8 * 1024
+  @range_header "bytes=0-#{@sniff_bytes - 1}"
+
+  describe "sniff_image_type/1 with a real image body" do
+    test "returns image/jpeg for a JPEG body" do
+      url = "https://example.com/media/image.jpg"
+      jpeg = File.read!("test/fixtures/image.jpg")
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: jpeg}
+      end)
+
+      assert {:ok, "image/jpeg"} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+
+    test "returns image/png for a PNG body" do
+      url = "https://example.com/media/image.png"
+      png = File.read!("test/fixtures/image.png")
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: png}
+      end)
+
+      assert {:ok, "image/png"} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+
+    test "returns image/gif for a GIF body" do
+      url = "https://example.com/media/image.gif"
+      gif = File.read!("test/fixtures/image.gif")
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: gif}
+      end)
+
+      assert {:ok, "image/gif"} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+  end
+
+  describe "sniff_image_type/1 sends a ranged request" do
+    test "sends a Range header asking for the first chunk" do
+      url = "https://example.com/media/ranged"
+      jpeg = File.read!("test/fixtures/image.jpg")
+
+      Tesla.Mock.mock(fn
+        %{method: :get, url: ^url, headers: headers} ->
+          headers_map = Enum.into(headers, %{})
+          assert headers_map["range"] == @range_header
+
+          %Tesla.Env{status: 200, body: jpeg}
+
+        %{method: :get, url: ^url} ->
+          # If the sniffer did not send the Range header, fail explicitly.
+          flunk("expected Range header to be sent with the request")
+      end)
+
+      assert {:ok, "image/jpeg"} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+  end
+
+  describe "sniff_image_type/1 with a non-image body" do
+    test "returns nil for plain text" do
+      url = "https://example.com/media/note.txt"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: "just some plain text, not an image at all"}
+      end)
+
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+
+    test "returns nil for an HTML body" do
+      url = "https://example.com/page"
+      html = File.read!("test/fixtures/rel_me_anchor.html")
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: html}
+      end)
+
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+
+    test "returns nil for an empty body" do
+      url = "https://example.com/media/empty"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: ""}
+      end)
+
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+
+    test "returns nil for a nil body" do
+      url = "https://example.com/media/nil-body"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: nil}
+      end)
+
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+  end
+
+  describe "sniff_image_type/1 on HTTP failure" do
+    test "returns nil on a 404" do
+      url = "https://example.com/media/missing"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 404, body: ""}
+      end)
+
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+
+    test "returns nil on a 500" do
+      url = "https://example.com/media/oops"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 500, body: "server error"}
+      end)
+
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+
+    test "returns nil on a 3xx (not in 200..299)" do
+      url = "https://example.com/media/redirect"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 302, body: ""}
+      end)
+
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+
+    test "returns nil on a transport-level error" do
+      url = "https://example.com/media/econnrefused"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        {:error, :econnrefused}
+      end)
+
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+
+    test "returns nil when the HTTP adapter raises" do
+      url = "https://example.com/media/raises"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        raise "boom"
+      end)
+
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(url)
+    end
+  end
+
+  describe "sniff_image_type/1 guard clauses" do
+    test "returns nil for nil input without making a request" do
+      # No Tesla mock is configured; if a request were made, Tesla.Mock would
+      # raise and fail the test.
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type(nil)
+    end
+
+    test "returns nil for an empty-string URL without making a request" do
+      assert {:ok, nil} = AttachmentTypeSniffer.sniff_image_type("")
+    end
+  end
+end
diff --git a/test/pleroma/web/activity_pub/object_validators/attachment_validator_test.exs b/test/pleroma/web/activity_pub/object_validators/attachment_validator_test.exs
index 744ae8704..870ad90cc 100644
--- a/test/pleroma/web/activity_pub/object_validators/attachment_validator_test.exs
+++ b/test/pleroma/web/activity_pub/object_validators/attachment_validator_test.exs
@@ -215,5 +215,226 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentValidatorTest do
         AttachmentValidator.cast_and_validate(attachment)
         |> Ecto.Changeset.apply_action(:insert)
     end
+
+    test "sniffs image/jpeg for octet-stream attachments whose body is an image" do
+      url = "https://example.com/media/no-extension/original"
+      jpeg = File.read!("test/fixtures/image.jpg")
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: jpeg}
+      end)
+
+      attachment = %{
+        "type" => "Document",
+        "mediaType" => "application/octet-stream",
+        "url" => [%{"type" => "Link", "href" => url, "mediaType" => "application/octet-stream"}]
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert [%{href: ^url, mediaType: "image/jpeg"}] = attachment.url
+    end
+
+    test "sniffs image/jpeg for attachments with a missing mediaType" do
+      url = "https://example.com/media/missing-type/original"
+      jpeg = File.read!("test/fixtures/image.jpg")
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: jpeg}
+      end)
+
+      attachment = %{
+        "type" => "Document",
+        "url" => [%{"type" => "Link", "href" => url}]
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert [%{href: ^url, mediaType: "image/jpeg"}] = attachment.url
+    end
+
+    test "leaves non-image octet-stream attachments as octet-stream" do
+      url = "https://example.com/media/some-document"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: "just some plain text, not an image at all"}
+      end)
+
+      attachment = %{
+        "type" => "Document",
+        "mediaType" => "application/octet-stream",
+        "url" => [%{"type" => "Link", "href" => url, "mediaType" => "application/octet-stream"}]
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert [%{mediaType: "application/octet-stream"}] = attachment.url
+    end
+
+    test "does not sniff when the remote already provided a real mediaType" do
+      # No Tesla mock is set up: if the validator tried to fetch, Tesla.Mock
+      # would raise and fail the test. A real image type must be kept as-is.
+      attachment = %{
+        "type" => "Document",
+        "mediaType" => "image/png",
+        "url" => [
+          %{"type" => "Link", "href" => "https://example.com/x", "mediaType" => "image/png"}
+        ]
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert [%{mediaType: "image/png"}] = attachment.url
+    end
+  end
+
+  describe "fix_media_type fallbacks and sniffing" do
+    test "uses mimeType as a fallback when mediaType is absent (real type, not sniffed)" do
+      # No Tesla mock: if the validator tried to fetch, the test would fail.
+      # A real image type coming from mimeType must be preserved as-is on the
+      # outer attachment and on the synthesised url entry (string url form).
+      attachment = %{
+        "type" => "Document",
+        "mimeType" => "image/png",
+        "url" => "https://example.com/x.png"
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert attachment.mediaType == "image/png"
+      assert [%{mediaType: "image/png"}] = attachment.url
+    end
+
+    test "prefers mediaType over mimeType when both are present" do
+      attachment = %{
+        "type" => "Document",
+        "mediaType" => "image/gif",
+        "mimeType" => "image/png",
+        "url" => "https://example.com/x.gif"
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert attachment.mediaType == "image/gif"
+      assert [%{mediaType: "image/gif"}] = attachment.url
+    end
+
+    test "sniffs when the url entry's mediaType is application/octet-stream and the body is an image" do
+      # The outer attachment mediaType is not sniffed (no top-level href); only
+      # the url entry is. We assert only on the url entry.
+      url = "https://example.com/media/mime-type-octet"
+      jpeg = File.read!("test/fixtures/image.jpg")
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: jpeg}
+      end)
+
+      attachment = %{
+        "type" => "Document",
+        "mediaType" => "application/octet-stream",
+        "url" => [%{"type" => "Link", "href" => url, "mediaType" => "application/octet-stream"}]
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert [%{mediaType: "image/jpeg"}] = attachment.url
+    end
+
+    test "sniffs when mediaType is the empty string and the body is an image" do
+      url = "https://example.com/media/empty-mediatype"
+      png = File.read!("test/fixtures/image.png")
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 200, body: png}
+      end)
+
+      attachment = %{
+        "type" => "Document",
+        "mediaType" => "",
+        "url" => [%{"type" => "Link", "href" => url, "mediaType" => ""}]
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert [%{mediaType: "image/png"}] = attachment.url
+    end
+
+    test "falls back to the declared type when the sniffer HTTP request fails" do
+      url = "https://example.com/media/sniff-404"
+
+      Tesla.Mock.mock(fn %{method: :get, url: ^url} ->
+        %Tesla.Env{status: 404, body: ""}
+      end)
+
+      attachment = %{
+        "type" => "Document",
+        "mediaType" => "application/octet-stream",
+        "url" => [%{"type" => "Link", "href" => url, "mediaType" => "application/octet-stream"}]
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert [%{mediaType: "application/octet-stream"}] = attachment.url
+    end
+
+    test "sniffs each url entry independently" do
+      jpeg_url = "https://example.com/media/a"
+      octet_url = "https://example.com/media/b"
+      jpeg = File.read!("test/fixtures/image.jpg")
+
+      Tesla.Mock.mock(fn
+        %{method: :get, url: ^jpeg_url} ->
+          %Tesla.Env{status: 200, body: jpeg}
+
+        %{method: :get, url: ^octet_url} ->
+          %Tesla.Env{status: 200, body: "definitely not an image"}
+      end)
+
+      attachment = %{
+        "type" => "Document",
+        "mediaType" => "application/octet-stream",
+        "url" => [
+          %{"type" => "Link", "href" => jpeg_url, "mediaType" => "application/octet-stream"},
+          %{"type" => "Link", "href" => octet_url, "mediaType" => "application/octet-stream"}
+        ]
+      }
+
+      {:ok, attachment} =
+        attachment
+        |> AttachmentValidator.cast_and_validate()
+        |> Ecto.Changeset.apply_action(:insert)
+
+      assert [
+               %{href: ^jpeg_url, mediaType: "image/jpeg"},
+               %{href: ^octet_url, mediaType: "application/octet-stream"}
+             ] = attachment.url
+    end
   end
 end