I have been unable to reproduce this when uploading files manually, nor have I seen this in any environment other than beta cluster commons.
When running the automated test for UW for Firefox only against beta commons, UW starts to upload a .png file, the thumbnail is there, but the upload never completes.
Error Console shows the message
Timestamp: 04/18/2013 01:37:53 PM
Error: TypeError: result is null
Source File: http://commons.wikipedia.beta.wmflabs.org/w/load.php?debug=false&lang=en&modules=ext.uploadWizard&skin=vector&version=20130412T222614Z&*
I am not completely sure that the production issue and the browser test issue are related, but it seems possible at least.