Returns the suffix of a filename. If the file has no suffix, returns None.
  Can return an empty string if the filenam ends with a period.

def getsuffix(subject):
  """
  Returns the suffix of a filename. If the file has no suffix, returns None.
  Can return an empty string if the filenam ends with a period.
  """

  index = subject.rfind('.')
  if index > subject.replace('\\', '/').rfind('/'):
    return subject[index+1:]
  return None