The first problem I see is that the forms encoding type is not set to mulipart/form-data
<form id="xl_form" method="post" action="">
<form action="" method="post" enctype="multipart/form-data" id="xl_form">
If your still having problems after changing the encoding type, add the following at line 1 to see if a php error is occurring:
and on the bindings panel, expand the WA Uploaded Files Global Status group and add the Debug Info binding to the page. Post back the debug info after trying to upload a file.