It looks like Brotli is a de-facto standard (RFC 7932 is an informational RFC), and it says the IANA registry has been updated. However, this appears to be http-only, with no corresponding program used for compression of files, and thus no "file extension" to be registered, or program name to put in the encodings_map table. We don't have most of the types listed in the http content encodings registry in mimetypes, so I don't at this time see any reason to add br. mimetypes is aimed at files, not http. In what context do you want to use mimetimes to look up br?