Register APNG MIME type

The newest git HEAD of MIME already knows about APNG, but this
hasn’t been released yet. Without this, APNG attachments from
remote posts won’t display as images in frontends.

Fixes: akkoma#657

config/config.exs

@@ -167,7 +167,10 @@
   "application/xrd+xml" => ["xrd+xml"],
   "application/jrd+json" => ["jrd+json"],
   "application/activity+json" => ["activity+json"],
-  "application/ld+json" => ["activity+json"]
+  "application/ld+json" => ["activity+json"],
+  # Can be removed when bumping MIME past 2.0.5
+  # see https://akkoma.dev/AkkomaGang/akkoma/issues/657
+  "image/apng" => ["apng"]
 }
 
 config :mime, :extensions, %{

test/pleroma/web/activity_pub/object_validators/attachment_validator_test.exs

@@ -11,6 +11,23 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AttachmentValidatorTest do
   import Pleroma.Factory
 
   describe "attachments" do
+    test "works with apng" do
+      attachment =
+        %{
+          "mediaType" => "image/apng",
+          "name" => "",
+          "type" => "Document",
+          "url" =>
+            "https://media.misskeyusercontent.com/io/2859c26e-cd43-4550-848b-b6243bc3fe28.apng"
+        }
+
+      assert {:ok, attachment} =
+               AttachmentValidator.cast_and_validate(attachment)
+               |> Ecto.Changeset.apply_action(:insert)
+
+      assert attachment.mediaType == "image/apng"
+    end
+
     test "works with honkerific attachments" do
       attachment = %{
         "mediaType" => "",