Uploading file in updatepanel

.aspx

 <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
 <asp:FileUpload ID="flupload" runat="server" />    
                <asp:Button ID="btnupload" runat="server" Height="30px"
                    onclick="btnupload_Click"
                    style="margin-left:100px;background-color:rgb(9, 16, 198);color:White;"
                    Text="Upload" Width="120px" />
 </ContentTemplate>
        <Triggers>
            <asp:PostBackTrigger ControlID="btnupload" />
        </Triggers>
    </asp:UpdatePanel>