Add thumbnail_remote_url in MediaAttachment REST response (#14358)

* Add thumbnail_remote_url in MediaAttachment REST response

* Change thumbnail_remote_url to preview_remote_url

app/serializers/rest/media_attachment_serializer.rb

@@ -4,7 +4,7 @@ class REST::MediaAttachmentSerializer < ActiveModel::Serializer
   include RoutingHelper
 
   attributes :id, :type, :url, :preview_url,
-             :remote_url, :text_url, :meta,
+             :remote_url, :preview_remote_url, :text_url, :meta,
              :description, :blurhash
 
   def id
@@ -35,6 +35,10 @@ class REST::MediaAttachmentSerializer < ActiveModel::Serializer
     end
   end
 
+  def preview_remote_url
+    object.thumbnail_remote_url.presence
+  end
+
   def text_url
     object.local? ? medium_url(object) : nil
   end