AjaxFileUpload update page content in UploadComplete Event

I have an AjaxFileUpload, I want after file upload complete, a label on page shows the filename just uploaded.

I have code like this:

<asp:UpdatePanel runat="server" ID="UpdatePanel" UpdateMode="Always"><ContentTemplate><asp:Label ID="Label2" runat="server" Text="123"></asp:Label><asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" /><ajaxToolkit:AjaxFileUpload ID="AjaxFileUpload1"
    AllowedFileTypes="jpg,jpeg,gif,png"
    MaximumNumberOfFiles="1"
    runat="server" onuploadcomplete="AjaxFileUpload1_UploadComplete"/></ContentTemplate><Triggers><asp:PostBackTrigger ControlID="AjaxFileUpload1"/><asp:PostBackTrigger ControlID="Button1" /></Triggers></asp:UpdatePanel>

I have code to update Label's text in AjaxFileUpload1_UploadComplete and Button1_Click.

After a file is upload, no changes on Label2.Text (I am pretty sure that AjaxFileUpload1_UploadComplete is fired and handled).

I click Button1, Label2.Text is updated.

So what could I do to let fileuploader to update label2?