> However this seems to compete with the functionality of the types map so another consideration is content-types_map where the content-type is the key and the pair values are lists of valid filename extensions:

There is mimetypes.MimeTypes().types_map_inv

> https://docs.python.org/3.8/library/mimetypes.html#mimetypes.MimeTypes.types_map_inv
> Tuple containing two dictionaries, mapping MIME types to a list of filename extensions: the first dictionary is for the non-standards types and the second one is for the standard types. They are initialized by common_types and types_map.

>>> mimetypes.MimeTypes().types_map_inv[1]['text/html']
['.htm', '.html']