Python 3.6 gets some of those "wrong":
https://bugs.python.org/issue1043134

This has been fixed in Python 3.7:
python/cpython#14375

The override dict has been generated by copying the types_map from the
current git master:
https://github.com/python/cpython/blob/v3.10.0a3/Lib/mimetypes.py#L414-L547

And then running the following with Python 3.6:

    import mimetypes

    reverse = {}
    for ext, mimetype in types_map.items():
        if mimetype not in reverse:
            reverse[mimetype] = ext
    assert reverse['text/plain'] == '.txt'

    for mimetype, ext in reverse.items():
        got = mimetypes.guess_extension(mimetype)
        if got != ext:
            print(f'    "{mimetype}": "{ext}",  # not {got}')

Python 3.6 gets some of those "wrong":
https://bugs.python.org/issue1043134

This has been fixed in Python 3.7:
python/cpython#14375

The override dict has been generated by copying the types_map from the
current git master:
https://github.com/python/cpython/blob/v3.10.0a3/Lib/mimetypes.py#L414-L547

And then running the following with Python 3.6:

    import mimetypes

    reverse = {}
    for ext, mimetype in types_map.items():
        if mimetype not in reverse:
            reverse[mimetype] = ext
    assert reverse['text/plain'] == '.txt'

    for mimetype, ext in reverse.items():
        got = mimetypes.guess_extension(mimetype)
        if got != ext:
            print(f'    "{mimetype}": "{ext}",  # not {got}')

Python 3.8 slightly changed how the `mimetypes` module is initialized
(python/cpython#14375), and this interacted
badly with the reinitialization code in `zope.contenttype` in such a way
as to make the extensions in `lp.services.mime` be ineffective.  Work
around this in a way that works with both old and new versions of Python
by monkey-patching `mimetypes.init`.