File upload withing updatepanel dosent work even on PostBackTrigger

Hi,
I have a file upload control in a update panel, and I am aware that it needs full post back to upload a file as partial post back will not do. I have added a PostBackTrigger to the trigger section. Still not able to upload a file.
Here is he html
<Triggers>                                 <asp:AsyncPostBackTrigger ControlID="btnUpload" EventName="Click" />                                 <asp:PostBackTrigger ControlID="btnSubmit" />< ...