Avoid close the ModalPopupExtender - asp.net

Question

I have a GridView which charge user data, selecting one lifts the panel through a ModalPopupExtender, this panel has UpdatePanel nested as you can see in the code, pressing the save button closes the Popup, can not find the error. help me with this problem.
Removing the UpdatePanel not let the FileUpload upload the file but no longer the Popup is closed, but I need to upload the file and that the Popup is not closed.
<asp:Panel ID="pDatos" runat="server" BackColor="#E0E0E0">
<table>
<tr>
<td>
<asp:GridView runat="server" ID="gvResultado" AutoGenerateColumns="false"
OnRowCommand="gvResultado_OnRowCommand" DataKeyNames="Id, Estado" >
<Columns>
<asp:ButtonField DataTextField="Id" ItemStyle-ForeColor="Black"
ControlStyle-CssClass="hover" CommandName="gvResultado"
HeaderText="Verificar"/>
<asp:BoundField DataField="Nombre" HeaderText="Nombre" ItemStyle-Width="320px" />
<asp:BoundField DataField="Observaciones" HeaderText="Observaciones" ItemStyle-Width="360px" />
<asp:BoundField DataField="Fecha" HeaderText="Fecha" />
<asp:BoundField DataField="Estado" HeaderText="Estado" ItemStyle-Width="120px" />
</Columns>
</asp:GridView>
</td>
</tr>
</table>
</asp:Panel>
<asp:HiddenField ID="hfPopup" runat="server" />
<ajax:ModalPopupExtender ID="mpePopup" runat="server" PopupControlID="pPopup"
TargetControlID="hfPopup" BackgroundCssClass="backgroundColor">
</ajax:ModalPopupExtender>
<asp:Panel ID="pPopup" runat="server" BackColor="#CFCFCF" Style="display: none" >
<asp:UpdatePanel ID="upPopup" runat="server">
<ContentTemplate>
<table style="width:740px;" align="center">
<tr>
<td align="center" colspan="2">
<asp:GridView runat="server" ID="gvHistorial" AutoGenerateColumns="false"
OnRowCommand="gvHistorial_RowCommand" DataKeyNames="Descargar" >
<Columns>
<asp:BoundField DataField="Legajo" HeaderText="Legajo" ItemStyle-Width="100"/>
<asp:BoundField DataField="Observaciones" HeaderText="Observaciones" ItemStyle-Width="550" />
<asp:BoundField DataField="Fecha" HeaderText="Fecha" ItemStyle-Width="150"/>
<asp:ButtonField DataTextField="Descargar" HeaderText="Descargar" CommandName="gvHistorial_Descargar" />
</Columns>
</asp:GridView>
</td>
</tr>
<tr>
<td><!-- --></td>
<td >
<asp:UpdatePanel ID="upArchivo" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:FileUpload ID="luSeleccione" runat="server" />
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="bGuardar" />
</Triggers>
</asp:UpdatePanel>
</td>
</tr>
<tr>
<td><!-- --></td>
<td align="left">
<asp:Button runat="server" ID="btnGuardar" OnClick="btnGuardar_Click"
Text="Guardar" CssClass="Button hover" />
<asp:Button runat="server" ID="btnSalir" OnClick="btnSalir_Click"
Text="Salir" CssClass="Button hover" />
</td>
</tr>
</table>
</ContentTemplate>
</asp:UpdatePanel>
</asp:Panel>