Steps to reproduce:

1. use the upload dialog to upload a file (in my case it was huwiki -> commons)
2. customize the Name field to Foo.png
3. upload

Expected result: the file is called Foo.png

Actual result: the file is called Foo.png.png and you get a confusing error which makes it seem like the file wasn't uploaded at all.

Seen on https://commons.wikimedia.org/wiki/File:Editors_on_huwiki_until_2019-04-01.png.png