WoW, Egypt Winnnnnnnn,[:D][:D]
and I just found the solution for that problem
try this if you want

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<script language="javascript">
     function ValidateFile(source, args)
     {
        try
         {
        var fileAndPath=
           document.getElementById(source.controltovalidate).value;
        var lastPathDelimiter=fileAndPath.lastIndexOf("\\");
        var fileNameOnly=fileAndPath.substring(lastPathDelimiter+1);
        var file_extDelimiter=fileNameOnly.lastIndexOf(".");
        var file_ext=fileNameOnly.substring(file_extDelimiter+1).toLowerCase();
        if(file_ext!="jpg")
             {
             args.IsValid = false;
             if(file_ext!="gif")
               args.IsValid = false;
                  if(file_ext!="png")
                  {
                    args.IsValid = false;
                     return;
                  }
                }
        }catch(err)
        {
          txt="There was an error on this page.\n\n";
          txt+="Error description: " + err.description + "\n\n";
          txt+="Click OK to continue.\n\n";
          txt+=document.getElementById(source.controltovalidate).value;
          alert(txt);
          }
         
           args.IsValid = true;
    }
    </script>
      
 <asp:FileUpload ID="FileUpload1" runat="server" />

<asp:CustomValidator ID="CustomValidator1"
ClientValidationFunction="ValidateFile"  runat="server"
ControlToValidate="FileUpload1"
Display="dynamic" ErrorMessage="images only ">
</asp:CustomValidator>

please post comments if there any comment,I'm not sure about it yet,

Thank you