<cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" PopupControlID="Panl1" TargetControlID="Button1"
    CancelControlID="Button2" BackgroundCssClass="Background">
</cc1:ModalPopupExtender>

<asp:Panel ID="Panl1" runat="server" CssClass="Popup" align="center" style = "display:none">
<asp:Button ID="Button2" CssClass="button" runat="server" Text="Закрыть" />
    <div id="block" class="slider slider1">
            <div class="sliderContent">
                <%foreach (var file in ViewBag.Files)
                { 
                %>
                <div class="item">
                    <img src="/Content/slider/profgr_general/<%: file.title%>" alt="" />
                </div>
                <%
                }
                %>
            </div>
        </div>
</asp:Panel>

 protected void ChangeViewBag(object sender, EventArgs e)
    {
        msk_labourmarket_asp_.Models.DataContainer db = new msk_labourmarket_asp_.Models.DataContainer();
        ViewBag.Files = db.Files.Where(x => x.company_id == Convert.ToInt32(this.CompDD.SelectedValue)).ToList();
    }