I'd be very careful with this. We get hack attempts daily.
If I understand this, you would like them to fill out the contact form and then add a file from their computer? If so, this file will have to be saved somewhere on the web server in order to be mailed out.
I'd think twice about allowing just anyone to upload files to your server. Maybe in the contact form page, you just write "Please email <blah blah> form to [email protected]? it's not good to put email addresses directly on your web page as spiders come and grab them and give them to spammers, but I like it better than the other alternative.
Or, you could do this. Create the contact form and have it go to [email protected]. Create an autoreply from this email address that says, "if you are new please send form <blah blah> to [email protected] . This way, you keep it pretty secure and accomplish the file transfer without adding it to your server.
...hope this helps.
oh, and if i'm not understanding your question and was just babbling....sorry about that.