Effect similar to the infamous bug 37073 and bug 54392, but different manifestation. No browser provided, but we need to be proactive because affected users are typically unlikely to look for us (especially if it's Internet Explorer again).
https://commons.wikimedia.org/w/index.php?title=Commons:Upload_Wizard_feedback&diff=prev&oldid=117334466 (report by well known user)
https://commons.wikimedia.org/?oldid=117334991 (successful upload some minutes later)
Version: master
Severity: major
Whiteboard: aklapper-moreinfo
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=65030