Since you have used File Upload to save the file to the server, in the attachments section of universal email, you wont be able to use the Submited File option.
Instead, select the Enter value option for the attachment, and click the lightning bolt icon.
From the listing of data bindings, select the WA Uploaded File (your file field) binding group and select Server Directory binding, in the code window, select the server directory binding code, the select the server file name binding and click ok.
Now for the entered value, place your cursor before the Server File name binding code and past the server directory binding code in front of it.