ok, to troubleshoot, make the following changes to your page:
1) turn on error reporting. Add the following code at line 1:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>
2) add the file upload debug info to the body of the page. go to the bindings panel, and expand the WA Uploaded Files Global Status group and add the Debug info binding to the page
3) write the session and File contents to the page.
add the following code just before the closing </body> tag:
<pre><?php var_dump($_FILES); ?></pre>
<pre><?php var_dump($_SESSION); ?></pre>