Add basic support for remote attachments with multiple media types (#34996)

app/lib/activitypub/parser/media_attachment_parser.rb

@@ -15,7 +15,7 @@ class ActivityPub::Parser::MediaAttachmentParser
   end
 
   def remote_url
-    url = Addressable::URI.parse(@json['url'])&.normalize&.to_s
+    url = Addressable::URI.parse(url_to_href(@json['url']))&.normalize&.to_s
     url unless unsupported_uri_scheme?(url)
   rescue Addressable::URI::InvalidURIError
     nil
@@ -43,7 +43,7 @@ class ActivityPub::Parser::MediaAttachmentParser
   end
 
   def file_content_type
-    @json['mediaType']
+    @json['mediaType'] || url_to_media_type(@json['url'])
   end
 
   private