I just had the same problem!
I had to download AVG free from AVG Anti-Virus and Internet Security - Welcome and it picked up over 1000 infected files and viruses!! even though I subscribe to norton antivirus and it picked up nothing.
Once you've scanned, re-boot and try to upload something again. This time pay close attention to the end of the file name as this virus changes mp3 files to zip files with a exe. extension. Re-boot again and you will see that your files are no longer changing.

Good luck to you and a curse of pox on the geeks that create the viruses.
(I think it's a replacement for something they're missing)