We have simple HTML form with , like shown below:
This is an example that could work for you if what you need is not exactly the path, but a reference to the file working offline.
http://www.ab-d.fr/date/2008-07-12/
It is in french, but the code is javascript :)
This are the references the article points to: http://developer.mozilla.org/en/nsIDOMFile http://developer.mozilla.org/en/nsIDOMFileList