Hi, zero!
Thank you for reporting this bug! Unfortunately, I couldn't find the problem in the forum software that causes it (the sourceforge.net servers have rather limited PHP capabilities), so I disabled attachments for now and opened the directory that contains the current attachments for direct downloads:
http://gnomecc.sourceforge.net/forum/attachments/Until someone is able to fix this problem, one has to copy'n'paste his themes into this forum or upload them elsewhere

Has anyone a better hosting service available for this project?
Jack ;-)