ModalPopupExtender dont show preview image

hello friends, can anyone give me a tip, I want to load an image in a FileUpload control which is inside a panel in a ModalPopupExtender, I'm using a script to view the image before uploading it, but I can not see the preview image, not 'running in modal, anyone know how to fix this

this is my code

<div class="box">
    <asp:Image ID="Image1" runat="server" Height="120px" Width="120px" ImageUrl="~/img_sistema/sin_foto.jpg" />
    <div class="mask">
        <asp:Button ID="btnShowPopup" runat="server" Style="display: none" />
        <a id="btnShow" runat="server" href="javascript:void(0)" class="link" title="Cambiar Logo"
            onclick="Showpopup()"></a>
    </div>
    <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnShowPopup"
        PopupControlID="pnlpopup" CancelControlID="btnHide" BackgroundCssClass="modalbackground">
    </asp:ModalPopupExtender>
    <asp:Panel ID="pnlpopup" runat="server" BackColor="#FFFFFF" Height="205px" Width="400px"
        Style="display: none; border: 10px solid groove;">
        <div>
            <a href="javascript:void(0)" id="btnHide" onclick="Hidepopup()" style="float: right;
                background-color: #F7FE2E;">Cerrar</a>
            <br />
            <asp:Label ID="Label2" runat="server" Text="Cambiar Logo." Style=""></asp:Label>
            <br />
            <br />
            <asp:FileUpload ID="FileUpload1" runat="server" onchange="showimagepreview(this)" />
            <br />
            <asp:Image ID="imgprvw" runat="server" Width="120px" Height="100px" ImageUrl="~/img_sistema/sin_foto.jpg" />
            <br />
            <asp:Button ID="Button1" runat="server" Text="Guardar" />
        </div>
    </asp:Panel>
</div>

script preview image

<script type="text/javascript">
    function showimagepreview(input) {
        if (input.files && input.files[0]) {
            var filerdr = new FileReader();
            filerdr.onload = function (e) {
                $('#imgprvw').attr('ImageUrl', e.target.result);
            }
            filerdr.readAsDataURL(input.files[0]);
        }
    }
</script>

show modal

<script type="text/javascript">
    //Function to Hide ModalPopUp
    function Hidepopup() {
        $find('ModalPopupExtender1').hide();
    }
    //Function to Show ModalPopUp
    function Showpopup() {
        $find('ModalPopupExtender1').show();
    }
</script>

thanks for all