I think this is an HTTP/HTML question, about the upload form element (I forget what it's called). It lets your browser grab a file and POST it (as Multi-Part) to the server.
There is no way to control what the browser does with it. Netscape, for example does (or used to) always pop the browse dialog in some default (netscape install) directory looking for *.html files. Every time. Very annoying.
You are going to need a signed
applet to do something more.