Hide zip and rar files from guests

[Sean James]

Ive been looking but cant seem to find a hack, that would only hide zip files and maybe rar files from guests.

Would be good for forums that have files hosted within posts, but would like to show a screenshot to guests, but have them register before they can download the file.

For example:
http://www.bluepearl-design.com/foru...-template.html

The attached thumbnails will be shown, but the attached files will require registration before downloading.