To do this most easily you would use an upload extension like Digital File Pro to handle uploading the file to the server, and use Universal Email to add that uploaded file as an attachment.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.