I just noticed this today. I am writing an email wherein part of my message was to inform/let my recipient see my attached screenshots. The thing is I forgot to attach the images and instead immediately hit the "Send" button. Luckily, gmail prompts me to attach the file.
Attaching file/s in email but forgetting to do so has always been a problem to some/most users like me. Good thing, Google made a way to solve this simple problem. Hmmnn.. have they employed real time text analytics in resolving this problem as what they do with targeted advertising? Whatever it is, I must say good job to Google's ingenuity!
By the way, I've tried replicating this in Yahoo or Facebook but hitting the Send/Post button would just automatically send the message eventhough the files were not yet attached.