Asp.net - Increassing Session timeout only file downloading

My requirement

In ASP.NET session management

For my website timeout = 20 mins and i have written the code to redirect once the session was expired, the code is working perfect, the actual client requirement was eventhough during session timeout , suppose if the file is downloading session must be active.In web.config i have used FORM Authentication even though i used sliding expiration = true , the session will continue but finaly unable to download the file. Please give an better solun .