What to do? Wait until the recipient emails us back "Where is the file?", issue a resend with a short apology? Pretend it didn't happen?
To make things worse, without the attachment, the send transaction is near-instantaneous; there's little chance of clicking "cancel" in time.
Issue a resend with an apology. We are only human after all. We are permitted, from time to time, to make mistakes, and it is to our advantage to 'fess up and make amends before putting the other party to the trouble of sending us a reply.
If you find you are doing this once a week or more frequently, stop sending emails. Save the email instead.
Get into a better habit, that of closing the email and saving it in your "output" or "drafts" box, to be sent an hour later.
Truth is, there are not many emails that need to be sent instantaneously.
Saving as a draft and re-reading of an hour later before resending can save you a lot of embarrassment AND make you look more professional.
Along the same lines, when I find I can't keep up with email, I do not send personal emails until after 5pm.
If nothing else, it reduces chatter ion my mailbox during prime business hours.