To make these edits you need to go into WA_Unisersal_Email > Templates > contactus.php and update the code on near line 10 that looks like this:
if ($pval != "" && strpos($pkey,"Security_") !== 0 && $pkey != "x" && $pkey != "y") {
In here you need to add an exception for the file field you have, you would add it in like this:
if ($pval != "" && strpos($pkey,"Security_") !== 0 && $pkey != "x" && $pkey != "y" && strpos($pkey,"<your file field name here>") !== 0") {
Next you will need to add in a reference to the server file name of the file you uploaded, you should be able to just drag and drop the binding for the uploaded file onto this page, it should look like this for the server file name:
<?php echo $WA_DFP_UploadStatus["WA_UploadResult1"]["serverFileName"]; ?>
If you have any trouble with this post back with the contactus template file and your modified contact form page and let us know what the trouble is.