Don't you mean 'uploaduser.php' in the admin directory? That's the link that shows when clicking Users -> Accounts -> Upload Users.
There are only these 'upload.php' files in a 1.9.19+:
./mod/assignment/upload.php
./mod/exercise/upload.php
./mod/workshop/upload.php
uploaduser.php file has the following required:
require('../config.php');
require_once($CFG->libdir.'/adminlib.php');
require_once($CFG->libdir.'/csvlib.class.php');
require_once($CFG->dirroot.'/user/profile/lib.php');
require_once('uploaduser_form.php');
If your hosting provider says 'calling file on another server', did they happen to mention the location/url of that server?
'spirit of sharing', Ken