docs: def resolve_format(format, path):
      """Looks at a file's extension and format (if any) and returns format.
      """
      if format is None:
          if (re.match(r'.+\.(yml|yaml)$', path)):
              return 'yaml'
          elif (re.match(r'.+\.tsv$', path)):
              return 'tsv'
      else:
          return format.lower()