docs: def get_image_files(directory, files):
      """Recursively iterate through directory tree and list all files that have a
      valid image file suffix
  
      Parameters
      ----------
      directory : directory
          Path to directory on disk
      files : List(string)
          List of file names
  
      Returns
      -------
      List(string)
          List of files that have a valid image suffix
      """
      # For each file in the directory test if it is a valid image file or a
      # sub-directory.
      for f in os.listdir(directory):
          abs_file = os.path.join(directory, f)
          if os.path.isdir(abs_file):
              # Recursively iterate through sub-directories
              get_image_files(abs_file, files)
          else:
              # Add to file collection if has valid suffix
              if '.' in f and '.' + f.rsplit('.', 1)[1] in VALID_IMGFILE_SUFFIXES:
                  files.append(abs_file)
      return files