Assume test.JPG already exists. Then while trying to upload test.jpg, clicking "next" during the last step only brings you back to the "describe" step without an error message. Seems like test.JPG and test.jpg are treated as two different file names for most of the process, but not in the last step?
This is actually a duplicate of T130242, but that didn't have reproduction steps, so – thank you! I finally connected the dots and that issue is fixed now (there's an error message explaining the problem and the user doesn't get stuck).
But yeah, we should detect this earlier. I'll look into this.
This has been fixed and the fix will be deployed to Wikimedia Commons on Wednesday, 27 April 2016, with MediaWiki version 1.27.0-wmf.22, per https://www.mediawiki.org/wiki/MediaWiki_1.27/Roadmap.