ArticleNotePageValidator: Fix when attachments are a Map (ie. owncast)

lib/pleroma/web/activity_pub/object_validators/article_note_page_validator.ex

@@ -65,6 +65,11 @@ defp fix_replies(%{"replies" => replies} = data) when is_bitstring(replies),
 
   defp fix_replies(data), do: data
 
+  def fix_attachments(%{"attachment" => attachment} = data) when is_map(attachment),
+    do: Map.put(data, "attachment", [attachment])
+
+  def fix_attachments(data), do: data
+
   defp fix(data) do
     data
     |> CommonFixes.fix_actor()
@@ -72,6 +77,7 @@ defp fix(data) do
     |> fix_url()
     |> fix_tag()
     |> fix_replies()
+    |> fix_attachments()
     |> Transmogrifier.fix_emoji()
     |> Transmogrifier.fix_content_map()
   end