If you're lucky, it's in the Outlook Secure Temp folder, normally a
subfolder of Temporary Internet Files. Here's how to find it: See Step 1 on http://www.howto-outlook.com/faq/securetemp.htm . Enter the path contained
in the registry key into the Address field of Windows Explorer and click the
Go button. You'll then be able to see inside the Secure Temp folder and
hopefully your file will be there.
Best practice is to save attachments to disk before modifying them.