As stated by others (and the documentation), the path is not dependably returned by the user's browser. So, if you're really lucky, you might be able to find this in $filename = param('uploaded_file');. Of course, this assumes that the user actually uploads a file. Probably not what you want but it is a faint possibility.

What I've done to get around a similar problem is to have (intranet) users start the process with a local program (VBS in my case) which gets the required info from the local system and opens an internal webpage with the appropriate parameters to do the work.