Check File Uploads for Viruses

I am currently developing a social networking site using RoR that uses
the attachment_fu plugin to handle image uploads. I also want to
enable users to upload photoshop files, but am worried about security
vulnerabilities because I want other users to be able to download
these photoshop files. What is to stop someone from uploading a
malicious file (i.e. virus) just by changing the extension? I don't
want someone downloading a file with a virus on it from my site. Is
there any way to screen files for viruses when they are uploaded in
RoR? Any thoughts? Thanks.