Http error on media upload using Google App Engine

I am using WP 3.9.1 , installed on the Google App Engine (https://emumbawp.appspot.com/) – When I upload media, I get an HTTP error and an error code of 500 in the response. I installed plugin “google-app-engine”(https://ww.wp.xz.cn/plugins/google-app-engine/) which supporrs media upload but still having the problem. I reviewed my file modules/uploads.php but no conflict there. Any suggestions?

Snapshot here