Google’s GMail is kind of nice in that regards. It checks the email that you are sending and if it sees the phrase “is attached” it makes the basic assumption that you are about to attach something to the email. If you have no attachments when you click “Send” GMail then pops up the warning you can see here.
Of course, if you want to tell your mother that the receipt for the payment you just made for her “is attached to the letter you sent”, then you will also get the warning popping up. Still, its nice to know someone is checking.