Use headers for file content type recognition.

lib/pleroma/upload.ex

@@ -9,9 +9,8 @@ def store(%Plug.Upload{} = file) do
     File.cp!(file.path, result_file)
 
     # fix content type on some image uploads
-    matches = Regex.named_captures(~r/\.(?<ext>(jpg|jpeg|png|gif))$/i, file.filename)
-    content_type = if file.content_type == "application/octet-stream" and matches do
-      if matches["ext"] == "jpg", do: "image/jpeg", else: "image/#{matches["ext"]}"
+    content_type = if file.content_type == "application/octet-stream" do
+      get_content_type(file.path)
     else
       file.content_type
     end
@@ -61,4 +60,30 @@ defp upload_path do
   defp url_for(file) do
     "#{Web.base_url()}/media/#{file}"
   end
+
+  def get_content_type(file) do
+    # PNG: 89 50 4E 47 0D 0A 1A 0A
+    # GIF: 47 49 46 38 37 61
+    # GIF: 47 49 46 38 39 61
+    # JPEG: FF D8 FF DB
+    # JPEG: FF D8 FF E0 ?? ?? 4A 46 49 46 00 01
+    # JPEG: FF D8 FF E1 ?? ?? 45 78 69 66 00 00
+    match = File.open(file, [:read], fn(f) ->
+      case IO.binread(f, 8) do
+        <<0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a>> ->
+          "image/png"
+        <<0x47, 0x49, 0x46, 0x38, _, 0x61, _, _>> ->
+          "image/gif"
+        <<0xff, 0xd8, 0xff, _, _, _, _, _>> ->
+          "image/jpeg"
+        _ ->
+          "application/octet-stream"
+      end
+    end)
+
+    case match do
+      {:ok, type} -> type
+      _e -> "application/octet-stream"
+    end
+  end
 end