When using System.Web.Mail you can only attach files that reside on the local server. If you have an interface similar to Gmail, Yahoo, etc when the user clicks the "Attach" button this should be a FileUpload control where they can select files from their local machine.
When the user presses 'Send' the file should be uploaded to the server, attached to the email, the email should be sent, and the file that was uploaded should be deleted.
Read this if you want to know how to get a correct reply for your question:
^^Took that from planoie's profile^^
^^Modified text taken from gbianchi profile^^
Technical Editor for: Professional Search Engine Optimization with ASP.NET
Why can't Programmers, program??