I think this has been fixed in Python 3.7+ via https://github.com/python/cpython/pull/14375 - at least for a couple of types.
Comparing Python 3.6 with the current state, the following changed (which can be used as an "override" dict before calling mimetypes.guess_extension):
"application/manifest+json": ".webmanifest", # not None
"application/octet-stream": ".bin", # not .a
"application/postscript": ".ps", # not .ai
"application/vnd.ms-excel": ".xls", # not .xlb
"application/vnd.ms-powerpoint": ".ppt", # not .pot
"application/wasm": ".wasm", # not None
"application/x-hdf5": ".h5", # not None
"application/xml": ".xsl", # not .rdf
"audio/mpeg": ".mp3", # not .mp2
"image/jpeg": ".jpg", # not .jpe
"image/tiff": ".tiff", # not .tif
"text/html": ".html", # not .htm
"text/plain": ".txt", # not .bat
"video/mpeg": ".mpeg", # not .m1v