File attachment process

I have been working on a project for work where I'm changing the file attachment process in squirrelmail. I've got most of it worked out but I'm stuck in a certain area. I'll try to explain it clearly.

User1: Writes an e-mail and adds an attachment. By doing this the file is put /var/spool/squirrelmail/attach/ and named by GenerateRandomString(32, '', 7). He has not sent yet so at this point /var/spool/squirrelmail/attach/ contains 1 file (3543eknohgsogjfdnsdfh)

User2: Writes an e-mail and adds an attachment. Their attachment does the same thing but with a different random string. Now /var/spool/squirrelmail/attach/ has 2 files. (3543eknohgsogjfdnsdfh and aklfas5435jn6as)

User1: Clicks send and somehow squirrelmail knows to only get 3543eknohgsogjfdnsdfh and not the other. How is this done? What part of the code tells it what files names to look for in /var/spool/squirrelmail/attach/ during the send process.