Welcome to JavaRanch.
The download of a file is under the control of the browser (and the user controlling it) - neither an
applet nor JavaScript can get at the location where the user stored the file.
During upload, the browser might use the same path as default that it used when the last file was downloaded, but that's not guaranteed (and browser-specific to begin with).
I think you'll have to depend on the user to navigate to the file.