Gmail team today added a new experiment to Gmail Labs called Forgotten Attachment Detector. With this new experiment, Gmail would keep an eye on what you type in e-mail message. And if you mention of attaching file in the e-mail text and forget to attach file and click on Send, it’ll give you an alert giving you another chance to add an attachment before sending e-mail. Since the functioning of this experiment depends on content of e-mail message, it might not work sometimes e.g. if you type in "file attached", it doesn’t work but works for "file is attached".