considering you've set up a forum where you allow all, I take it as you have done all the neccessary settings in Admin-Forums. (using the checkboxes etc to allow users to upload)
but it sounds like you have something wrong in the mod, did you use the zip file linked from the first post aswell, and added all fixes written in the first post? (it sounds like either attach.php is faulty, or there are more than one line missing), considering only admins can post/see files, it implies that it's something with the if statements and/or calls to functions in attach.php to check if user is allowed or not...
I'm quite occupied tomorrow, but in the evening I should probably find some time to do some hunting
If you like, you can compress the php files, and I can take a peak on them ... (in that case, mail me an url to the archive, to frank_n05pam@hotmale.com, and replace hotmale with msn's mail service )
Edit: Saw that you had mailed that the mail didn't work, but it did work, only that the hotmail spam filter had put the mail in junk mail, but I saw it and replied ... (then I saw the mail from this email-form on PunBB that you couldn't mail, but it did work ... must have been because it was delivered into the junk mail box or something, further emails should go to my inbox)
Edit: I have recieved the files, and when I unpacked the zip PunBB weren't installed correctly, at leat what it seems, so the problems is most probably there. I'm actually amazed stuff worked at all, to be honest. Hopefully moving all files to correct position will make it all work (and there was some files missing aswell, or at least "include/attach.php")
Edit: got another file, everything looks ok to me, no idea what might be wrong...
Edit: and so does the database ... I'm now fresh out of ideas ...
Edit: We have to give up, there's something that I cannot find, and a fully working version on apache fail on IIS, if it's apache vs IIS I'm not sure, as admins can upload, so it might be a combination of IIS+PHP, but I'm not sure.... either way, the user having these problem could let people be administrators.