I solved this too (do you need someone for customer support at Web Assist? LOL!).
In the email attachment dialog box I selected the "uploads/[FileName]: Server File Name" option from bindings panel and added the "upload/"string before the binding value.
So, now the "enter value" field looks like:
uploads/<?php echo $WA_DFP_UploadStatus["WA_UploadResult1"]["WA_UploadResult1_1"]["serverFileName"]; ?>