'private', 'premium_only', 'subscriber_only', 'needs_auth',
'unlisted' or 'public'. Use 'InfoExtractor._availability'
to set it
+ media_type: The type of media as classified by the site, e.g. "episode", "clip", "trailer"
_old_archive_ids: A list of old archive ids needed for backward compatibility
_format_sort_fields: A list of fields to use for sorting formats
__post_extractor: A function to be called just before the metadata is