def METHOD_NAME() -> typing.Set[str]:
    """
    Return supported extensions as a set of strings.
    Plain extensions only, no pattern!
    """
    types: typing.Set[str] = set()
    for filetype in filetypes.keys():
        cls = get_dataset_cls(filetype)
        types = types.union({ext.lower() for ext in cls.get_supported_extensions()})
    return types