Audrey-
If I understand you correctly, you want to allow the users of your site to attach a file to an eamil. And I assume that this file would come from their hard-drive. Are these assumptions correct?...if so, TCP/IP has no real bearing on this. You are right, in that you should brose the files, and upload them to the server. Once there, you can attach and send them. I have used a class called mime_mail, originally written by Sascha Schumann, and modified (cleaned up) by Tobias Ratschiller--unfortunately, I can't for the life of me, remember where I found this little gem. If you cant find it, drop me a line, and Ill email it to ya.
Hope this helps
Darren