Try this for fancy looks :
System.IO.FileInfo[] fi = new System.IO.DirectoryInfo(path).GetFiles().
Where(f => !(f.FullName.EndsWith(".mp3") || f.Name.EndsWith(".txt"))).ToArray();
or if you want to get involved with regex
System.IO.FileInfo[] fi = new System.IO.DirectoryInfo(path).GetFiles().
Where(s => !System.Text.RegularExpressions.Regex.IsMatch(s.Name, @"\.(txt|mp3|png)$")).ToArray();
Credits to
OriginalGriff