Desiging custom forms with outlook

Hello,

I have this next problem which i'm trying to solve...i want to design a form starting from the ?post? form in Outlook 2003. In this form I plan to add one file in attachments and then post this form. The problem I have is that I want the subject field of this form to be automatically completed with the name of the file i add in attachments. Does anybody knows how can I do this?
Thank you very much for your time,
Cosmin