how to transfer files in chat application in asp.net c#

I have created the chate application from the below link
http://www.codeproject.com/Articles/17617/Create-a-Chat-System-using-Ajax-and-ASP-NET

Now i want to transfer files from this application. i.e. if 2 user are online and they want to send & receive any files(can be image files, .txt files, .doc files etc) as we can do in yahoo messenger... I have gone through many sites but i didnt get any useful stuff..

How can i do this????