File Stream C#

using System.IO;
using System.Collections.Generic;
using System.Windows.Forms;
using System.Text.RegularExpressions;
public class Utils
{
    public static List GetValidFiles(List FileList, string sValidExt)
    {
        return FileList.FindAll(sFileName => sValidExt.Contains(new FileInfo(sFileName).Extension.ToLower()));
    }
}