Pop-up and download zip file in ASP.NET

Question

I've a webform wich present a pop-up with a confirmation button :

<div id="Popup" class="modal hide">
    <asp:Panel ID="MyPanel" runat="server">
        <asp:UpdatePanel ID="UpdatePanelPopup" runat="server" UpdateMode="Conditional">
            <Triggers>
                <asp:PostBackTrigger ControlID="ButtonConfirmerPopup" />
            </Triggers>
            <ContentTemplate>
                <div class="modal-header">
                    <button type="button" class="close" data-dismiss="modal">×</button>
                    <h3 id="PopupTitle" runat="server">
                        <asp:Label runat="server" ID="LibelleTitrePopup"></asp:Label></h3>
                </div>
                ....
                <div class="modal-footer">
                    <a href="#" class="btn" data-dismiss="modal">Annuler</a>
                    <asp:Button ID="ButtonConfirmerPopup" ValidationGroup="Submit" runat="server" OnClick="ButtonConfirmerPopup_Click"
                        CssClass="btn btn-success" Text="Confirmer" />
                </div>
            </ContentTemplate>
        </asp:UpdatePanel>
    </asp:Panel>
</div>

The click button executes :

protected void ButtonConfirmerPopup_Click(object sender, EventArgs e)
{
    int idPart = Convert.ToInt32(HiddenFieldIdStagiairePopup.Value);

    if (Convert.ToBoolean(HiddenFieldIsValidationPopup.Value))
    {
        bool result = Valider(idPart);

        if (!result)
            return;

        GetZipDocumentFormation();
        Bootstrap.Message(MessageContainer, "Inscription validée", "alert-success");
        BindListPart();
        UpdatePanelGrid.Update();
        MyPanel.Visible = false;
    }
    else
        Refuser(idPart);
    ButtonConfirmerPopup.Attributes.Remove("data-dismiss");
}

And the ZIP file creation (using ICSharpCode.SharpZipLib.Zip):

private void GetZipDocumentFormation()
{
    if (ViewState["DocumentsFormation"] == null)
        return;

    var pjs = (List<string>)ViewState["DocumentsFormation"];
    ViewState["DocumentsFormation"] = null;

    // envoi des fichiers en ZIP
    const string fileName = "documents_formation.zip";
    string root = Server.MapPath("~/Resources/");
    Zip.ZipFiles(pjs, string.Format("{0}\\{1}", root, fileName));

    //HttpResponse response = HttpContext.Current.Response;
    //response.ClearContent();
    //response.Clear();
    //response.ContentType = "text/plain";
    //response.AddHeader("Content-Disposition", "attachment; filename=" + fileName);
    //response.TransmitFile(string.Format("{0}\\{1}", root, fileName));
    //response.Flush();

    Response.AppendHeader("content-disposition", "attachment; filename=" + fileName);
    Response.ContentType = "application/zip";
    Response.WriteFile(string.Format("{0}\\{1}", root, fileName));


    HttpContext.Current.ApplicationInstance.CompleteRequest();
}

The zip creation is OK, i get the file. But the pop-up is always displayed and the UpdatePanel is not updated. If I don't execute zip file creation, everything is OK.

Could you please purpose me help ?

Regards.