When the filename on the local computer has an invalid/bad character (like a leading space in my case) the UW stops the upload with a non helpful "unknown-warning".
The old upload form shows at least the better message
'badfilename' => 'Filename has been changed to "$1".',
and I can retry with the changed filename.
I think the UW should change the filename silently and continue with the upload, maybe notifying the user in the last step about the changed filename.
Version: master
Severity: normal