Mar 26, 2012 11:29 PM|davebettin|LINK
You could create a message handler to intercept the request and return the file. Read more about handlers here: http://www.asp.net/web-api/overview/working-with-http/http-message-handlers.
You will want to make sure to create the response, add your file stream, and return it. Just this alone is naive, but it should help you get started.