#2 is an interesting hack indeed. But from the demo video (posted in a different comment), it also works for attachment so I don't think he is using this method.
Edit: I signed up and you were right. He uses a remote image. It works when I email using the unsend.it web app but doesn't work if I send it through the Gmail website so I couldn't test the attachment.
Edit: I signed up and you were right. He uses a remote image. It works when I email using the unsend.it web app but doesn't work if I send it through the Gmail website so I couldn't test the attachment.