request time out while file uploading

hi all,
im trying to upload a 8mb of pdf file but im getting Request Timeout error, below code i have mentioned in the web.config file. In my web page i have used AJAX Scriptmanager, Updatepanel.

<httpRuntime maxRequestLength="25600"/> below is the error message
Server Error in '/' Application. Request timed out. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Request timed out.

Source Error:

An unhandled exc ...