File Stream C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
public static class Utility
{
    public static string GetClearExtension(string filePath)
    {
        return String.IsNullOrEmpty(filePath)
            ? null
            : Path.GetExtension(filePath).Substring(1).ToLower();
    }
}