My boss forgot to attach a file he has talked about in his mail. How do I respond to his mail asking for the attachment?
The email did not have the TPS Report template attached. Could you please send it to me?
This should do. Don't overthink this. You seem to be worried that the boss may get "offended" for pointing out his mistake, but that worry is unnecessary. This isn't a big deal, your boss will most likely send you the attachment with a comment, "Oops, sorry I forgot to attach it", and move on as if nothing happened.
Alternatively, you could ask the boss over phone/IM or just walk up to his office, but there really is no problem with sending the email.