Uploading Webextensions .xpi to AMO

I have an Webextensions .xpi (ported from Chrome) which works in Firefox 48, but when I submit to AMO it gets rejected with a “Your add-on failed validation with 1 error. Received an empty response from the server; status: 403”. Is uploading Webextensions working yet? I can install the unsigned .xpi no problem.

Submission should work correctly. If you continue getting this error, please file a new bug report and attach the XPI.