for a file to upload, the encoding type of the form must be sret to multipart/form-data.
change the form tag to;
<form action="" method="post" enctype="multipart/form-data" name="application" id="application" onsubmit="WAValidateRQ(document.application.Name,'- Please enter your name.',document.application.Name,0,false,'text');WAValidateRQ(document.application.Email,'- Please enter your email address.',document.application.Email,0,false,'text');WAValidateRQ(document.application.Phone_Number,'- Please enter your phone number.',document.application.Phone_Number,0,false,'text');WAValidateRQ(document.application.Date_of_Birth,'- Please enter your date of birth.',document.application.Date_of_Birth,0,false,'text');WAAlertErrors('The following errors were found','Correct invalid entries to continue',true,false);return document.MM_returnValue">