docs: def suggest_filename(metadata):
  	"""Generate a filename like Google for a song based on metadata.
  
  	Parameters:
  		metadata (~collections.abc.Mapping): A metadata dict.
  
  	Returns:
  		str: A filename string without an extension.
  	"""
  
  	if 'title' in metadata and 'track_number' in metadata:  # Music Manager.
  		suggested_filename = f"{metadata['track_number']:0>2} {metadata['title']}"
  	elif 'title' in metadata and 'trackNumber' in metadata:  # Mobile.
  		suggested_filename = f"{metadata['trackNumber']:0>2} {metadata['title']}"
  	elif 'title' in metadata and 'tracknumber' in metadata:  # audio-metadata/mutagen.
  		track_number = _split_number_field(
  			list_to_single_value(
  				metadata['tracknumber']
  			)
  		)
  		title = list_to_single_value(metadata['title'])
  
  		suggested_filename = f"{track_number:0>2} {title}"
  	else:
  		suggested_filename = f"00 {list_to_single_value(metadata.get('title', ['']))}"
  
  	return _replace_invalid_characters(suggested_filename)