string[] filePaths = Directory.GetFiles(@"c:TEMP");
foreach (string filePath in filePaths)
{
    if (filePath.Contains(".bmp"))
        File.Delete(filePath);
}

Directory.GetFiles(@"C:TEMP", "*.bmp");

string[] t = Directory.GetFiles(Environment.CurrentDirectory, "*.pdf");
    Array.ForEach(t, File.Delete);

string[] t = Directory.GetFiles(Environment.CurrentDirectory, "*.txt");
    Array.ForEach(t, File.Delete);