for the file to upload, the forms encoding type must set to multipart/form-data
change the form tag:
<form action="<?php echo $_SERVER["PHP_SELF"]; ?>" method="post" name="SimpleContact_Cool_Default" class="Cool_Default" id="SimpleContact_Cool_Default">
to:
<form action="<?php echo $_SERVER["PHP_SELF"]; ?>" method="post" enctype="multipart/form-data" name="SimpleContact_Cool_Default" class="Cool_Default" id="SimpleContact_Cool_Default">