I don't really understand the purpose of this script. The user enters their own email address and the path to a file. The script then sends a copy of that file to the user's email address. Why?
At any rate, it seems like the easiest solution would be to allow the user to open up a mail client (like messenger that comes with communicator in Linux) and attach files that way.
Either you are going to have to get into the nitty gritty of how multipart forms send stuff and read it and fix some problems that even (bows head in reverence)Lincoln Stein has not solved yet, or you are going to have to allow the user to upload the file to the server.
It'd be easier to write a web page with pretty pictures that would explain how to do this than to write a script that acts as a mediator.