It looks like the wrong binding was used for the attachment. I think the attachment line should be:
$Email->addAttachment("attachments_folder/".$WA_DFP_UploadStatus["WA_UploadResult1"]["WA_UploadResult1_1"]["serverFileName"] ."");
What kind of form validation are you using on the page? You should be able to make it required, but how you do that depends on the type of validation you are using to begin with.