Look for the DFP function: WA_DFP_UploadFiles on your page and for the forth parameters, you will need to change it to look something like this:
You can see that we are concatenating the ID to the file name. But to replace empty space with underscores, we will need to add this statement:
$_FILES['fileField']['name'] = str_replace(" ", "_", $_FILES['fileField']['name']);
before the DFP Upload server behavior.