you are using the upload file behavior to upload the file to the server?
if this is the case, on the attachments tab, select to use the entered value option for the attachment. Click the lightning bolt icon and select the WAS Uploaded File (File field Name) -> Server Directory binding. Copy the code for the server directory, then select the Server File name binding. in the code window that shows the server file name, place your cursor before the server file name binding and paste the code for the server directory in front of it.