From Add-in Express:
Outlook Attachment Alarm will never let you forget to add an attachment if the body of the message implies it.
The add-in is very easy to use and requires no hard tuning. You just specify the words and phrases that imply an attachment and add them to the Key words list. Outlook Attachment Alarm scans all outgoing messages and if it finds words or phrases from the Key words list, it raises the alarm and suggests inserting the attachment.