问文件上传不能在asp.net中使用updatepanel

EN

<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>