更新面板内的asyncfileupload不会触发上传方法

Question

我在更新面板中有asyncfileupload，我想在选择文件时上传文件一次，相同的代码在更新面板之外运行良好，但我的设计是这样的，我可以把它放在更新面板之外，我该怎样才能完成这项工作

  <asp:UpdatePanel ID="UP_DDL" runat="server" UpdateMode="Always" Visible="true"
                    RenderMode="Inline">
      <ContentTemplate>  
   <asp:asyncfileupload id="AFU_Doc" runat="server"
     uploadingbackcolor="Yellow" onuploadedcomplete="btnDocUpload_Click"
      onclientuploadcomplete="sa" throbberid="spanUploading2" />
       <span id="spanUploading2" runat="server">Uploading...</span>
      </ContentTemplate>
         <Triggers>
            <asp:AsyncPostBackTrigger ControlID="AFU_Doc" />
         </Triggers>
  </asp:UpdatePanel>