the form encodeing type must be set to multipart/form-data in order for a file to be attached.
change:
<form id="ContactForm" name="ContactForm" method="post" action="upload.php">
to:
<form action="upload.php" method="post" enctype="multipart/form-data" name="ContactForm" id="ContactForm">
NOTE: you have 2 instances of the UE behavior on this page, make sure that the first one is configured to attach the file.