Add a list of files with a reference to a list of objects.

def add(self, files, items):
        """
        Add a list of files with a reference to a list of objects.
        """
        if isinstance(files, (str, bytes)):
            files = iter([files])
        for pathname in files:
            try:
                values = self._filemap[pathname]
            except KeyError:
                self._filemap[pathname] = items
            else:
                values.extend(items)