Can't be done. The ability to do that is deliberately not there because it would be a security problem. By design, the only "files" that a web site can automatically read from your hard drive are the ones that the web site wrote to your hard drive in the first place (and these are cookies).
By forcing the user to use the $_FILE box, it allows the user to be in control of the files that are being read from their computer. It's an extra step but it gives the user a chance to say, "Wait a minute, this web site shouldn't see that file!"