FileUpload Control Inside Update Panel Without Using Trigger(PostBackTrigger)

Hello Friend I need a help,
I am Using File Upload Control Inside Update Panel, I got the fileupload Control Null, But If i use Trigger then its Working fine. Like Following
<asp:UpdatePanel runat="server" ID="UpdatePanel"> <ContentTemplate> <div class="form-group col-lg-3"> <label>Add Image</label> <asp:FileUpload runat="server" ID="flUpload" CssClass="form-control" TabIndex="5" /> </div> <asp:Button runat="server" ID="btnSubmitTask" Text="SubmitTask" TabIndex="12" OnClick="btnSubmitTask_Click" /> </ContentTemplate> <Triggers> <asp:PostBackTrigger ControlID="btnSubmitTask"/> </Triggers> </asp:UpdatePanel> But I ...