asp.net updatePanel PostBackTrigger triggering full postback

Question

Below is the markup page

  <asp:UpdatePanel runat="server" UpdateMode="Conditional">
        <ContentTemplate>
                    <table>
                        <tr>
                            <td>
                                <label>File name<label>
                            </td>
                        </tr>
                       <tr>
                            <td>
  <asp:TextBox runat="server" ID="txtName" Width="150%"></asp:TextBox>
                            </td>
                        </tr>
                      <tr>
                            <td>
                                <label>File</label>
                            </td>
                            <td>
                                <asp:FileUpload runat="server" ID="fileUpload" />
                            </td>
                        </tr>
                    </table>
                    <asp:Button runat="server" ID="btnUpload" Text="Upload file"  OnClick="btnUpload_Click" />
                </div>

                <div class="panel-heading">File display</div>
                <div class="panel-body">
     <asp:GridView runat="server">
       </asp:GridView>
                    <asp:Button runat="server" ID="btnRefresh" Text="Refresh" OnClick="btnRefresh_Click" />
                </div>
            </div>
        </ContentTemplate>
        <Triggers>
            <asp:PostBackTrigger ControlID="btnUpload" />
        </Triggers>
    </asp:UpdatePanel>

With the above code, whenever the user clicks "btnUpload", there'll be a post back, but there is an update panel to prevent the post back, when I change the PostBackTrigger to AsyncPostBackTrigger, the page didn't post back but the fileUpload.HasFile will be false and was unable to get the file.

What's wrong with the above code?Why the postBackTrigger triggers a post back?

Answer 1

File upload will not work with AsyncPostBackTrigger. It will only work with PostBackTrigger.
SOLUTION : If You want to upload file with AsyncPostBackTrigger than you have to use ajaxtoolkit. And in AjaxToolkit their is Control name with "AsyncFileUpload".
Hope You know how to add AjaxToolkit to your Toolbox. If you dont know than tell me i will guide you.