def get_region_filename(region, subregion):
    """Returns the filename needed to download/manage PBF files.

    Parameters
    ----------------------
    region : str
    subregion : str

    Returns
    ----------------------
    filename : str
    """
    base_name = '{}-latest.osm.pbf'
    if subregion is None:
        filename = base_name.format(region)
    else:
        filename = base_name.format(subregion)

    return filename