Can't upload a group avatar

Summary

I can not upload a avatar for groups. No difference between new group and editing an existing group. In both i can't upload a new avatar. The file-chooser window will be opened, i can select the file and click to open - but nothing happens then. File size is 160 KB, file-type is a png. Also tried it with an jpg.

Steps to reproduce

Create a new group or open an existing one, try to upload an avatar image.

Expected behavior

The avatar image which was choosed in the File-chooser should be uploaded.

Actual behavior

The selected file wouldn't be uploaded.

Output of checks

This bug happens on gitlab.com - Enterprise Edition 8.16.0-rc3-ee 4ec848c3