Hi,
In the file/script that is run after the form submission there will be something like this:
$mform = new mod_assignment_upload_form();
....
$data = $mform->get_data();
See for example mod/assignment/type/upload/upload.php. In $data object you will have access to all the fields set by the form, in your case $data->input and $data->output.
Tomasz (Tomek) Muras