Mail puts .html onto downloaded attachments

A guy I know has an iMac w/out the iSight and he said that he has emailed some .jpg and .xls files to himself from work. When he gets home and downloads them, mail adds .html to the downloaded files (filename.xls.html). I have never had this happen to me before so I really didnt know what to tell him, so I figured somebody here might know whats going on.

I will be out until late tonight (lighting fireworks and cooking) but ill be back on tonight, hopefully somebody can help. If not, its the 4th of July so I understand completely. Thanks for the help and Happy 4th of July!