diff --git a/changelog.d/mediaproxy-content-type.fix b/changelog.d/mediaproxy-content-type.fix
new file mode 100644
index 000000000..f36dd6283
--- /dev/null
+++ b/changelog.d/mediaproxy-content-type.fix
@@ -0,0 +1 @@
+Detect image content types for extensionless MediaProxy responses.
diff --git a/lib/pleroma/reverse_proxy.ex b/lib/pleroma/reverse_proxy.ex
index c7ee47c6e..061b2de9b 100644
--- a/lib/pleroma/reverse_proxy.ex
+++ b/lib/pleroma/reverse_proxy.ex
@@ -17,6 +17,7 @@ defmodule Pleroma.ReverseProxy do
   @max_read_duration :timer.seconds(30)
   @max_body_length :infinity
   @failed_request_ttl :timer.seconds(60)
+  @sniff_bytes 8 * 1024
   @methods ~w(GET HEAD)
 
   @allowed_mime_types Pleroma.Config.get([Pleroma.Upload, :allowed_mime_types], [])
@@ -59,7 +60,12 @@ 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
+
+  * `sniff_content_type` (default `false`): detects an image MIME type from the first
+  response chunk when the upstream type is missing or `application/octet-stream`.
 
   * `req_headers`, `resp_headers` additional headers.
 
@@ -69,11 +75,15 @@ defmodule Pleroma.ReverseProxy do
   @default_options [pool: :media]
 
   @inline_content_types [
+    "image/apng",
+    "image/avif",
+    "image/bmp",
     "image/gif",
     "image/jpeg",
     "image/jpg",
     "image/png",
     "image/svg+xml",
+    "image/webp",
     "audio/mpeg",
     "audio/mp3",
     "video/webm",
@@ -92,6 +102,7 @@ defmodule Pleroma.ReverseProxy do
           | {:req_headers, [{String.t(), String.t()}]}
           | {:resp_headers, [{String.t(), String.t()}]}
           | {:inline_content_types, boolean() | list(String.t())}
+          | {:sniff_content_type, boolean()}
           | {:redirect_on_failure, boolean()}
 
   @spec call(Plug.Conn.t(), String.t(), list(option())) :: Plug.Conn.t()
@@ -184,12 +195,14 @@ defmodule Pleroma.ReverseProxy do
   defp response(conn, client, url, status, headers, opts) do
     Logger.debug("#{__MODULE__} #{status} #{url} #{inspect(headers)}")
 
+    {headers, client, prefetched} = maybe_prefetch_for_content_type(headers, client, opts)
+
     result =
       conn
       |> put_resp_headers(build_resp_headers(headers, opts))
       |> streaming_compat
       |> send_chunked(status)
-      |> chunk_reply(client, opts)
+      |> chunk_reply(client, opts, prefetched)
 
     case result do
       {:ok, conn} ->
@@ -209,18 +222,28 @@ defmodule Pleroma.ReverseProxy do
     end
   end
 
-  defp chunk_reply(conn, client, opts) do
+  defp chunk_reply(conn, client, opts, :none) do
     chunk_reply(conn, client, opts, 0, 0)
   end
 
-  defp chunk_reply(conn, client, opts, sent_so_far, duration) do
-    with {:ok, duration} <-
-           check_read_duration(
-             duration,
-             Keyword.get(opts, :max_read_duration, @max_read_duration)
+  defp chunk_reply(conn, _client, _opts, :done), do: {:ok, conn}
+  defp chunk_reply(conn, _client, _opts, {:error, error}), do: {:error, error, conn}
+
+  defp chunk_reply(conn, client, opts, {:ok, data, duration}) do
+    with :ok <-
+           body_size_constraint(
+             byte_size(data),
+             Keyword.get(opts, :max_body_length, @max_body_length)
            ),
-         {:ok, data, client} <- client().stream_body(client),
-         {:ok, duration} <- increase_read_duration(duration),
+         {:ok, conn} <- chunk(conn, data) do
+      chunk_reply(conn, client, opts, byte_size(data), duration)
+    else
+      {:error, error} -> {:error, error, conn}
+    end
+  end
+
+  defp chunk_reply(conn, client, opts, sent_so_far, duration) do
+    with {:ok, data, client, duration} <- read_chunk(client, duration, opts),
          sent_so_far = sent_so_far + byte_size(data),
          :ok <-
            body_size_constraint(
@@ -235,6 +258,66 @@ defmodule Pleroma.ReverseProxy do
     end
   end
 
+  defp read_chunk(client, duration, opts) do
+    with {:ok, timer} <-
+           check_read_duration(
+             duration,
+             Keyword.get(opts, :max_read_duration, @max_read_duration)
+           ),
+         {:ok, data, client} <- client().stream_body(client),
+         {:ok, duration} <- increase_read_duration(timer) do
+      {:ok, data, client, duration}
+    end
+  end
+
+  defp maybe_prefetch_for_content_type(headers, client, opts) do
+    if Keyword.get(opts, :sniff_content_type, false) and generic_content_type?(headers) do
+      case read_chunk(client, 0, opts) do
+        {:ok, data, client, duration} ->
+          {maybe_put_image_content_type(headers, data), client, {:ok, data, duration}}
+
+        :done ->
+          {headers, client, :done}
+
+        {:error, error} ->
+          {headers, client, {:error, error}}
+      end
+    else
+      {headers, client, :none}
+    end
+  end
+
+  defp generic_content_type?(headers) do
+    headers
+    |> get_content_type()
+    |> String.trim()
+    |> String.downcase()
+    |> then(&(&1 in ["", "application/octet-stream"]))
+  end
+
+  defp maybe_put_image_content_type(headers, data) do
+    with false <- data == "",
+         {:ok, %{mime_type: "image/" <> _ = content_type}} <-
+           Majic.perform({:bytes, binary_part(data, 0, min(byte_size(data), @sniff_bytes))},
+             pool: Pleroma.MajicPool
+           ) do
+      [
+        {"content-type", content_type}
+        | Enum.reject(headers, fn {key, _} -> key == "content-type" end)
+      ]
+    else
+      _ -> headers
+    end
+  rescue
+    error ->
+      Logger.debug("#{__MODULE__}: content-type sniffing failed: #{Exception.message(error)}")
+      headers
+  catch
+    kind, reason ->
+      Logger.debug("#{__MODULE__}: content-type sniffing failed: #{kind}: #{inspect(reason)}")
+      headers
+  end
+
   defp head_response(conn, url, code, headers, opts) do
     Logger.debug("#{__MODULE__} #{code} #{url} #{inspect(headers)}")
 
@@ -281,17 +364,34 @@ defmodule Pleroma.ReverseProxy do
     |> Enum.filter(fn {k, _} -> k in @keep_req_headers end)
     |> build_req_range_or_encoding_header(opts)
     |> build_req_user_agent_header(opts)
-    |> Keyword.merge(Keyword.get(opts, :req_headers, []))
+    |> merge_headers(Keyword.get(opts, :req_headers, []))
+    |> maybe_force_identity_encoding(opts)
+  end
+
+  defp merge_headers(headers, extra_headers) do
+    Enum.reduce(extra_headers, headers, fn {key, value}, headers ->
+      List.keystore(headers, String.downcase(key), 0, {String.downcase(key), value})
+    end)
+  end
+
+  defp maybe_force_identity_encoding(headers, opts) do
+    if Keyword.get(opts, :sniff_content_type, false) do
+      List.keystore(headers, "accept-encoding", 0, {"accept-encoding", "identity"})
+    else
+      headers
+    end
   end
 
   # Disable content-encoding if any @range_headers are requested (see #1823).
   defp build_req_range_or_encoding_header(headers, _opts) do
     range? = Enum.any?(headers, fn {header, _} -> Enum.member?(@range_headers, header) end)
 
-    if range? && List.keymember?(headers, "accept-encoding", 0) do
-      List.keydelete(headers, "accept-encoding", 0)
-    else
-      headers
+    cond do
+      range? && List.keymember?(headers, "accept-encoding", 0) ->
+        List.keydelete(headers, "accept-encoding", 0)
+
+      true ->
+        headers
     end
   end
 
@@ -384,9 +484,33 @@ defmodule Pleroma.ReverseProxy do
 
       disposition = "attachment; filename=\"#{name}\""
 
-      List.keystore(headers, "content-disposition", 0, {"content-disposition", disposition})
+      replace_header(headers, "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
+
+        replace_header(headers, "content-disposition", disposition)
+      end
+    end
+  end
+
+  defp replace_header(headers, key, value) do
+    [{key, value} | Enum.reject(headers, fn {header, _} -> header == key end)]
+  end
+
+  defp inline_filename(content_type) do
+    case MIME.extensions(content_type) do
+      [ext | _] when ext != "" -> "inline.#{ext}"
+      _ -> nil
     end
   end
 
@@ -421,7 +545,9 @@ defmodule Pleroma.ReverseProxy do
     end
   end
 
-  defp check_read_duration(_, _), do: {:ok, :no_duration_limit, :no_duration_limit}
+  defp check_read_duration(_, _), do: {:ok, :no_duration_limit}
+
+  defp increase_read_duration(:no_duration_limit), do: {:ok, :no_duration_limit}
 
   defp increase_read_duration({previous_duration, started})
        when is_integer(previous_duration) and is_integer(started) do
diff --git a/lib/pleroma/web/mastodon_api/views/status_view.ex b/lib/pleroma/web/mastodon_api/views/status_view.ex
index 6949c5f2d..725dc2ce7 100644
--- a/lib/pleroma/web/mastodon_api/views/status_view.ex
+++ b/lib/pleroma/web/mastodon_api/views/status_view.ex
@@ -620,6 +620,7 @@ defmodule Pleroma.Web.MastodonAPI.StatusView do
         String.contains?(media_type, "image") -> "image"
         String.contains?(media_type, "video") -> "video"
         String.contains?(media_type, "audio") -> "audio"
+        attachment["type"] == "Image" -> "image"
         true -> "unknown"
       end
 
diff --git a/lib/pleroma/web/media_proxy/media_proxy_controller.ex b/lib/pleroma/web/media_proxy/media_proxy_controller.ex
index a0aafc32e..93f692008 100644
--- a/lib/pleroma/web/media_proxy/media_proxy_controller.ex
+++ b/lib/pleroma/web/media_proxy/media_proxy_controller.ex
@@ -208,6 +208,7 @@ defmodule Pleroma.Web.MediaProxy.MediaProxyController do
 
   defp media_proxy_opts do
     Config.get([:media_proxy, :proxy_opts], [])
+    |> Keyword.put_new(:sniff_content_type, true)
   end
 
   defp sandbox(conn, _params) do
diff --git a/test/pleroma/reverse_proxy_test.exs b/test/pleroma/reverse_proxy_test.exs
index ec4470379..ad1d75f33 100644
--- a/test/pleroma/reverse_proxy_test.exs
+++ b/test/pleroma/reverse_proxy_test.exs
@@ -222,6 +222,56 @@ defmodule Pleroma.ReverseProxyTest do
     assert Conn.get_resp_header(conn, "content-type") == ["application/octet-stream"]
   end
 
+  test "sniffs and preserves an image body with a generic content type", %{conn: conn} do
+    body = File.read!("test/fixtures/image.jpg")
+
+    ClientMock
+    |> expect(:request, fn :get, "/extensionless", headers, _, _ ->
+      assert {"accept-encoding", "identity"} in headers
+      {:ok, 200, [{"content-type", "application/octet-stream"}], %{body: body}}
+    end)
+    |> expect(:stream_body, fn %{body: ^body} = client ->
+      {:ok, body, Map.delete(client, :body)}
+    end)
+    |> expect(:stream_body, fn %{} -> :done end)
+
+    conn =
+      ReverseProxy.call(conn, "/extensionless",
+        sniff_content_type: true,
+        max_read_duration: :infinity,
+        req_headers: [{"accept-encoding", "gzip"}]
+      )
+
+    assert conn.resp_body == body
+    assert Conn.get_resp_header(conn, "content-type") == ["image/jpeg"]
+
+    assert Conn.get_resp_header(conn, "content-disposition") == [
+             "inline; filename=\"inline.jpg\""
+           ]
+  end
+
+  test "preserves a generic content type for a non-image body", %{conn: conn} do
+    body = "not an image"
+
+    ClientMock
+    |> expect(:request, fn :get, "/extensionless", _, _, _ ->
+      {:ok, 200, [], %{body: body}}
+    end)
+    |> expect(:stream_body, fn %{body: ^body} = client ->
+      {:ok, body, Map.delete(client, :body)}
+    end)
+    |> expect(:stream_body, fn %{} -> :done end)
+
+    conn = ReverseProxy.call(conn, "/extensionless", sniff_content_type: true)
+
+    assert conn.resp_body == body
+    assert Conn.get_resp_header(conn, "content-type") == ["application/octet-stream"]
+
+    assert Conn.get_resp_header(conn, "content-disposition") == [
+             "attachment; filename=\"extensionless\""
+           ]
+  end
+
   defp headers_mock(_) do
     ClientMock
     |> expect(:request, fn :get, "/headers", headers, _, _ ->
@@ -328,6 +378,24 @@ 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-disposition", "attachment; filename=\"duplicate.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 "with content-disposition header", %{conn: conn} do
@@ -340,6 +408,50 @@ 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 "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 "serves modern browser image types inline", %{conn: conn} do
+      disposition_headers_mock([
+        {"content-type", "image/webp"},
+        {"content-length", "0"}
+      ])
+
+      conn = ReverseProxy.call(conn, "/disposition")
+
+      assert Conn.get_resp_header(conn, "content-disposition") == [
+               "inline; filename=\"inline.webp\""
+             ]
+    end
   end
 
   describe "content-type sanitisation" do
diff --git a/test/pleroma/web/mastodon_api/views/status_view_test.exs b/test/pleroma/web/mastodon_api/views/status_view_test.exs
index 76123cd0f..faeca050e 100644
--- a/test/pleroma/web/mastodon_api/views/status_view_test.exs
+++ b/test/pleroma/web/mastodon_api/views/status_view_test.exs
@@ -668,6 +668,23 @@ defmodule Pleroma.Web.MastodonAPI.StatusViewTest do
       assert expected == StatusView.render("attachment.json", %{attachment: object})
       assert_schema(expected, "Attachment", api_spec)
     end
+
+    test "uses the ActivityStreams image type when the MIME type is generic" do
+      object = %{
+        "type" => "Image",
+        "url" => [
+          %{
+            "mediaType" => "application/octet-stream",
+            "href" => "https://example.com/extensionless"
+          }
+        ]
+      }
+
+      attachment = StatusView.render("attachment.json", %{attachment: object})
+
+      assert attachment.type == "image"
+      assert attachment.pleroma.mime_type == "application/octet-stream"
+    end
   end
 
   test "put the url advertised in the Activity in to the url attribute" do
diff --git a/test/pleroma/web/media_proxy/media_proxy_controller_test.exs b/test/pleroma/web/media_proxy/media_proxy_controller_test.exs
index f7e52483c..800159c21 100644
--- a/test/pleroma/web/media_proxy/media_proxy_controller_test.exs
+++ b/test/pleroma/web/media_proxy/media_proxy_controller_test.exs
@@ -71,7 +71,10 @@ defmodule Pleroma.Web.MediaProxy.MediaProxyControllerTest do
 
     test "it performs ReverseProxy.call with valid signature", %{conn: conn, url: url} do
       with_mock Pleroma.ReverseProxy,
-        call: fn _conn, _url, _opts -> %Conn{status: :success} end do
+        call: fn _conn, _url, opts ->
+          assert opts[:sniff_content_type]
+          %Conn{status: :success}
+        end do
         assert %Conn{status: :success} = get(conn, url)
       end
     end