fix 404s for missing static files

lib/pleroma/web/plugs/instance_static.ex

@@ -4,7 +4,7 @@
 
 defmodule Pleroma.Web.Plugs.InstanceStatic do
   require Pleroma.Constants
-  import Plug.Conn, only: [put_resp_header: 3]
+  import Plug.Conn, only: [put_resp_header: 3, put_status: 2, send_resp: 3, halt: 1]
 
   @moduledoc """
   This is a shim to call `Plug.Static` but with runtime `from` configuration.
@@ -51,10 +51,37 @@ defmodule Pleroma.Web.Plugs.InstanceStatic do
       |> Map.put(:from, from)
       |> Map.put(:content_types, false)
 
-    conn = set_content_type(conn, conn.request_path)
+    conn =
+      conn
+      |> set_content_type(conn.request_path)
+      |> Plug.Static.call(opts)
 
-    # Call Plug.Static with our sanitized content-type
-    Plug.Static.call(conn, opts)
+    if conn.halted do
+      conn
+    else
+      path = String.trim_leading(conn.request_path, "/")
+
+      if not File.exists?(file_path(path)) do
+        conn
+        |> put_status(:not_found)
+        |> send_404()
+        |> halt()
+      else
+        conn
+      end
+    end
+  end
+
+  defp send_404(conn) do
+    if String.ends_with?(String.downcase(conn.request_path), ".json") do
+      conn
+      |> put_resp_header("content-type", "application/json")
+      |> send_resp(404, Jason.encode!(%{error: "not found"}))
+    else
+      conn
+      |> put_resp_header("content-type", "text/plain")
+      |> send_resp(404, "Not found")
+    end
   end
 
   defp set_content_type(conn, "/emoji/" <> filepath) do