Not only is this frustrating, it can sometimes land you in very embarrassing situations.
Imagine your boss waiting for the sales report, you had sent the email message but forgot to include the attachment and left for the day.
There's some good hope for us forgetful souls. Matthew Baldwin has some simple but brilliant advice for us in Trade Tricks:
If you are sending an email with an attachment, add the attachment first, then compose the message, and then add email addresses to the send line.
Now there's no chance you'll have to send the ever-popular "whoops, forgot to attach the file" follow-up.
In fact, it's a good practice to always put the email addresses of the recipients in last, to ensure that an errant carriage return or mouse click won't fire off the message half-baked.