FileUpload looses file on PostBack in ASP.Net

I have Created a Web page with master page.

Page Contains two dropdown list

i am using Update panel with trigger.

when I select image using fileupload after state and city changes then record submitted with Image but when i select the image after that i did some changes with state and city on that time fileupload lost its selected file or files .

what I do so that file still remains after postback.