The Missing Attachment Powertoy fixes this:
"Have you ever forgotten to include an attachment to an important email? Missing Attachment Powertoy integrates with Microsoft Outlook 2007 and scans each message before it is sent, looking for words or phrases that would normally indicate an attachment. If it finds such references but the message does not have an attachment, it brings up a dialog to alert you and gives you the option to revise the message before sending. You can customize the list of trigger words based on your personal email habits and words that you would commonly use when sending a file attachment. Simple and useful."