validating a custom control

 i am using a custom control fol file upload (http://www.codeplex.com/fileuploadajax).
i want to apply a constraint that user must upload at least one file. with regular control like textbox it is quite easy to do. so try to get the name of file in a textbox. but somehow it is not working.
here is .aspx file :
<%@ Page Language="C#" AutoEventWireup="true" MasterPageFile="~/testMaster.master"
    CodeFile="Default.aspx.cs" Inherits="_Default" %>

<%@ Register Assembly="FUA" Namespace="Subgurim.Controles" TagPrefix="cc1" %>



<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
    test head
    <style type="text/css">
        .style1
       ...