文件输入发布的文件始终为空

Question

我正在尝试使用带有类型文件的输入上传文件，如图所示  

                    <div class="col-lg-12 col-sm-12 col-lg-xs col-md-12">
                       <table class="table-bordered table table-responsive"  >
                            <tr class=" row form-group ">
                                          <td class="text-center  col-lg-4 col-sm-4 col-xs-4 col-xl-4">
                                              <button onclick="triggerFileUpload()" class="btn   btn-social btn-twitter" style="border-radius: 17px">
                                                  <span class="fa fa-camera"></span>
                                                  <div class="text-center"> image</div>
                                              </button>
                                          </td>

                                      </tr>
                       </table>

                    </div>
                </div>

                <div class="form-group row">

                    <div class="col-lg-12 col-xs-12 col-sm-12 col-xs-12">    
                       <input style="visibility: hidden;" type="file" runat="server" onchange="readURL(this);" id="ftp" accept="image/*|audio/*;capture=camera,microphone" class="form-control" />

                        <img id="blah" class="img-responsive img center-block" style="display: none" runat="server" src="#" />

                    </div>

                </div> 

使用

正确显示blah中的所选文件

 <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>

以及更新面板中的所有内容

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

当我点击save button时，始终发布的文件为null，尽管我有页面加载

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