It's a bug in the extension it should be fixed in a future release.
if you do not want the file upload to be required, delete the line:
$WAFV_Errors .= WAValidateRQ((isset($_FILES["Contact_Form_group_Sample_FIle_Upload_MB_Max_size"])?$_FILES["Contact_Form_group_Sample_FIle_Upload_MB_Max_size"]["name"]:"") . "",false,5);