def file_prepender(filename):
    """
    Prepend an informational header on files managed by Molecule and returns
    None.

    :param filename: A string containing the target filename.
    :return: None
    """
    with open_file(filename, 'r+') as f:
        content = f.read()
        f.seek(0, 0)
        f.write(molecule_prepender(content))