Off the top of my head:
If you're getting people to upload to a mac and you're concerned that you're not getting the right file associations for double-clicking, then there's two fixes:
Firstly make sure people upload files with extensions (like 'PhotoshopFile.psd'). Doing that, your mac will use File Exchange to associate with the correct application.
Secondly, you could get the file type of the uploaded file. This can be ascertained in several ways, possibly including using the content-type of the upload. Once you get this you can either use MacPerl to change the file's Type and Creator to get your associations right or you could rename the file with an appropriate extension to get File Exchange to fix it.