Using GiveWP 2.12.2 and Donation Receipt Attachments for GiveWP 1.1.2, the tag {attachmenturl} does not display a link to the attachment. It just shows the raw tag.
Steps to replicate:
  1. have the aforementioned plugins installed
  2. upload and save a file in a single form in Donation Form Options >> Receipt Attachments
  3. make a test donation
  4. check email receipt and see that the raw tag is displayed