You might also want to take a look at the MIME type of the file upload, as well as the file extension using:

$type = $query->uploadInfo($filename)->{'Content-Type'};
[download]

Some OSs (pre-X Mac OS for example) rarely use file extensions to signify file types - so it's worth checking if you want to support those users.

My usual method is to believe the file extension if it's there, then back down to the Content-Type, then give up.