I accidentally ran into an automatic feature of Gmail the other day that I found useful, as it saved me from having to send a corrective e-mail. So, it turns out that if the body of your e-mail contains text that is indicative of your intent to attach a file, but you neglect to attach a file, then Gmail warns you of your potential omission.
After a little playing around, here are some phrases that trigger the warning:
"Find the attached"
"See the attached"
Here are some phrases that do not trigger the warning, but probably should:
"Attached a file"
"A file attached"
"Check out the attachment"