action="<?php echo (htmlentities($_SERVER["PHP_SELF"], ENT_QUOTES)); ?>"
It will appear two times on the page. That should fix the problem. The issue is that the form action isn't forwarding the $_GET variable you are using to filter the recordset. So after submitting the recordset isn't able to find the row with the files that need to be deleted.
By setting the action="" property, any parameters passed to the page are forwarded through the submit automatically.