How to do FileUpload

Hi,
I don't get filename in fileupload control at upload button first click,but I get filename in fileupload control at upload button second click,
My code
<asp:UpdatePanel runat="server" ID="UpdatePanel">                     <ContentTemplate>                         <asp:Button ID="btnUpload" runat="server" Text="Upload" onclick="btnUpload_Click" />                         <br />              &nbs ...