The files are stored in the Moodle data directory in a directory structure that humans can't work with. The actual paths to the files in that directory should not be used and its practically impossible anyway.
Instead use the files api and specify the context, component, file area, itemid and filename of the file to retrieve it. If you look around the poodllfilelib.php the code should help you figure it out, and you could add hooks in there to save the file elsewhere or to save the id of the file to your own database table for retrieval later.
I am afraid that you will just have to learn how to use the Moodle files api for this.