diff --git a/app/serializers/activitypub/note_serializer.rb b/app/serializers/activitypub/note_serializer.rb
index da99b9606..460a7ba7e 100644
--- a/app/serializers/activitypub/note_serializer.rb
+++ b/app/serializers/activitypub/note_serializer.rb
@@ -40,7 +40,7 @@ class ActivityPub::NoteSerializer < ActivityPub::Serializer
   end
 
   def source
-    { 'source' => object.proper.text, 'mediaType' => 'text/plain' }
+    { 'source' => object.proper.text, 'mediaType' => object.proper.content_type || 'text/plain' }
   end
 
   def content_map