See the $wgAntivirusSetup Talk Page for the details. I ran into the same issue as you did, and with some helpful brainstorming by the guys on the #mediawiki freenode channel, found it to be caused by clamdscan not having permission to read the temporary file!
Topic on User talk:Kghbln
Cool, I will give it a try soon. Yes the folks at the freenode channel a really good!