UpdatePanel, FileUpload in TabPage

Hi,

I have the following html structure in the designer:

<asp:UpdatePanel ID= "UpdatePanel1" runat="server">
<ContentTemplate>

    <asp:TabContainer ID="TabContainer1" runat="server"  CssClass="Tab" OnActiveTabChanged="TabContainer1_OnActiveTabChanged" AutoPostBack="true" >
        <asp:TabPanel runat="server" ID="TabPanel1" HeaderText="  Case  "    >
       </asp:TabPanel>

<asp:TabPanel runat="server" ID="TabPanelSynopsis" HeaderText="Synopsis">
<ContentTemplate>
    <asp:Panel ID="PanelSynopsis" runat="server">
        <uc4:ucPageSynopsis ID="ucPageSynopsis1" runat="server" />
    </asp:Panel>
<ContentTemplate>
</asp:TabPanel>
</asp:TabContainer>

ucPageSynopsis has a FileUpload control (which failed to work because of the UpdatePanel) and a button ButtonInsertSupportingDoc.  How can I put reference this button so that I can put it as a controlID in asp:PostBackTrigger in the webform?

<Triggers>
<asp:PostBackTrigger ControlID="ButtonInsertSupportingDoc" />
</Triggers>

</ContentTemplate>

Thanks,

tinac99
</asp:UpdatePanel>