Fileupload control not working with update panel

Question

<asp:ScriptManager ID="ScriptManager1" runat="server"> </asp:ScriptManager> <asp:UpdatePanel ID="UpdatePanel1" runat="server"> <ContentTemplate> <asp:Panel ID="pnlFirstRegistration" runat="server" DefaultButton="btnNext"> <asp:TextBox ID="txtFirstName" runat="server" class="txtbox"></asp:TextBox> <asp:Button ID="btnNext" runat="server" Text="NEXT" OnClick="btnNext_Click" /> </asp:Panel> <asp:Panel ID="pnlFinalRegistration" runat="server" DefaultButton="btnSubmit" Visible="false"> <asp:FileUpload ID="fileUploader" AllowMutliple="true" runat="server" accept=".png,.jpg,.jpeg,.gif" /> </div> <asp:Button ID="btnSubmit" runat="server" Text="SUBMIT" OnClick="btnSubmit_Click" /> </asp:Panel> </ContentTemplate> <Triggers> <asp:PostBackTrigger ControlID="btnSubmit" /> </Triggers> </asp:UpdatePanel>

 

I have a update panel where first is a Next button and then Submit. When clicking Submit it is showing fileload HasRows property false. What's the reason

 

It should show some files in upload control