You know how people always say that they're attaching a file, with words like "attached is" or "the following file(s)," etc.. And we've all been guilty of sending such messages *without* the attachments, right? It would be cool if there was a filter (similar to the one that places the targeted text ads) that springs into action when you hit the send button and have words/phrases like "attachment" "file" "this picture" "I'm sending you this..." and such things in the message.
I think it could be implemented similarly to the "You're sending this message without a subject, continue anyway?" feature that everyone has now. Just say, "Judging from the content of your e-mail, it seems like you want to attach a file, but you haven't specified any attachments. Attach files now?" (And there could be a checkable box saying "Don't ask me this again.")
What do you think? It wouldn't be perfect, since a computer program would be applying the instances, but it would probably save at least a few (million) people from embarassment.
Thanks for your time,
~ William Matheson