Javascript 文件输入已发布文件始终为空

我正在尝试使用输入上传文件，类型为file，如图所示

和所有在更新面板

 <asp:UpdatePanel ID="uy" runat ="server" UpdateMode="Always" >
                        <ContentTemplate >
     </ContentTemplate>
        <Triggers>
          <asp:PostBackTrigger ControlID ="btnsave" /> 
            <asp:PostBackTrigger ControlID="rblist" />
        </Triggers>
    </asp:UpdatePanel>

 <script>
                        function readURL(input) {

                            if (input.files && input.files[0]) {
                                var reader = new FileReader();

                                reader.onload = function (e) {
                                    $('#ContentPlaceHolder1_blah')
                                        .attr('src', e.target.result)
                                        //.width(150)
                                        .css({ display: "block" })
                                    //.height(200);

                                };

                                reader.readAsDataURL(input.files[0]);
                            }
                             initMap();
                        }


                    </script>

 <asp:UpdatePanel ID="uy" runat ="server" UpdateMode="Always" >
                        <ContentTemplate >
     </ContentTemplate>
        <Triggers>
          <asp:PostBackTrigger ControlID ="btnsave" /> 
            <asp:PostBackTrigger ControlID="rblist" />
        </Triggers>
    </asp:UpdatePanel>

 Page.Form.Attributes.Add("enctype", "multipart/form-data");