file upload has file always showing null even after using update panel

<asp:UpdatePanel runat="server" ID="updtsave" ChildrenAsTriggers="true" EnableViewState="true">
                           <ContentTemplate>
                               <asp:Button runat="server" ID="btnsave" OnClientClick="return ele()" Text="Save"
                                   CssClass="button" OnClick="btnsave_Click" />
                               <asp:Button runat="server" CssClass="button" ID="btnComplete" OnClientClick="return false"
                                   Text="Complete" OnClick="btnComplete_Click" />
                           </ContentTemplate>
                           <Triggers>
                               <asp:PostBackTrigger ControlID="btnsave" />
                           </Triggers>
                       </asp:UpdatePanel>