Herb- I'd say you've got the file types covered with
.JPG and .GIF for now but in the future other small size files allowable would be .txt, .doc, and acrobat files.

I could not ever see any video or music files as they are too large.

something else to think about is limiting max file size
and displaying the file size so a user will know what to expect as far as load time etc. No one likes surprises.

Have you considered some hashing routine to store the attachment files in folders by hashed names as function
of file name ? as the attachments begin to number in thousands that will be important.