Are they being filtered by exim on your cpanel box?
Do you have the &antivirus& line in full effect in your /etc/exim.conf ? Check out the filter file and see if those file extensions match up with what your client is trying to do.
Actually, I just found out that he cannot receive attachments from anybody that is NOT on the same server he is on. I've had this problem before but cannot remember how I fixed it. The mail cue in WHM is filling up.
Cant send attachments from horde but everything else is working fine. Can send the email but for some reason the attachment gets stripped. Anyone else have this trouble and have a resolution?