to troubleshoot the upload, add the following code at line 1:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
if(isset($_FILES)) var_dump($_FILES);
?>
then go to the bindings panel, and expand the WA Uploaded Files Global Status group and add the debug info binding to the page in the body section.
post back a copy of the page in a zip archive, and the results after attempting to upload a file.