Fix loading GIF media attachments, at least if it is being added as a literal attachment

lib/models/attachment_media_type_enum.dart

@@ -8,6 +8,10 @@ enum AttachmentMediaType {
       return AttachmentMediaType.unknown;
     }
 
+    if (text == 'gif' || text == 'gifv') {
+      return AttachmentMediaType.image;
+    }
+
     return AttachmentMediaType.values.firstWhere(
       (e) => e.name == text,
       orElse: () => unknown,