After deploying drupal-appengine-7.x-appengine-full-1.2 to GAE and enable Google App Engine in /admin/modules and add default bucket at /admin/config/media/file-system, the folder is created in the bucket properly for "Public file system path" and "Temporary directory". However, when trying to upload an image (on article creation page), the error "An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (30 MB) that this server supports" is displayed.
I'm not sure if it's really an issue with the module or some misconfiguration; however, I've seen some people asking questions about this issue w/o any answer. Therefore, I would like to create the ticket so that we may help more information about the issue and how to fix it.
P/S: We've also modified php.ini file to add upload_max_filesize and post_max_size with the value of "30M".