The realpath() functions takes a relative file name and converts it to a system path.
Click the Lightning bolt and select the server file name, this will add code in the attachments window similar to:
<?php echo $WA_DFP_UploadStatus["WA_UploadResult1"]["serverFileName"]; ?>
you will need to modify this to include the upload folder path:
<?php echo("upload/Folder/".$WA_DFP_UploadStatus["WA_UploadResult1"]["serverFileName"]); ?>
as well as the realpath() function:
<?php echo(realpath("upload/folder/".$WA_DFP_UploadStatus["WA_UploadResult1"]["serverFileName"])); ?>