input: Write a python function to get the file extension from a filename using the os module.
  assert get_filetype("example.jpg") == "jpg"
  assert get_filetype("document.PDF") == "pdf"
  assert get_filetype("archive.tar.gz") == "gz"
output: def get_filetype(filename):
      import os
      return os.path.splitext(filename)[1][1:].lower()