PostBackTrigger and UpdateProgress

hi everybody,

i'm having problems with updateprogress, i'm using a Postbacktrigger to load my excel file. I tried Asyncpostback but the file upload doesn't work with that. Everythings works fine except for updateprogress. (I know it can't work with PostbackTrigger.. but i absolutely need it, cause my file is huge and it takes a long time to load. I can't use a fixed timeout). Now i'm trying to start updateprogress onclientclick but.. i don't know how to stop it! I would stop it when file upload ends. Any ideas? THANK YOU!!!!

this is what i'm using to start Updateprogress:

<asp:UpdatePanel runat="server" ID="updButton" >
<ContentTemplate>
<asp:ImageButton runat="server" ID="provaexcel" ImageUrl="~/Images/ImmagineExcel.png"
OnClick="provaexcel_Click" OnClientClick="postbackButtonClick()" />
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="provaexcel" />
</Triggers>

</asp:UpdatePanel>

function postbackButtonClick() {
updateProgress = $find("<%= UpdateProgress15.ClientID %>");
window.setTimeout("updateProgress.set_visible(true)", updateProgress.get_displayAfter());
return true;
}

<asp:UpdatePanel runat="server" ID="provaupd15">
<ContentTemplate>

<div class="indicator" id="divclassProgress15" runat="server">
<asp:UpdateProgress ID="UpdateProgress15" runat="server" AssociatedUpdatePanelID="updButton"
DisplayAfter="0" DynamicLayout="true">
<ProgressTemplate>
<table class="loading">
<tbody>
<tr>
<td>

<asp:Image runat="server" ID="Comm_ind12" ImageUrl="~/App_themes/common/background/indicator.gif" />
<asp:Label CssClass="label" ID="Comm_porgress12" runat="server">
Attendere prego ......
</asp:Label>
</td>
</tr>
</tbody>
</table>
</ProgressTemplate>

</asp:UpdateProgress>
</div>

</ContentTemplate>
</asp:UpdatePanel>