I’ve been one to argue that additions to the mimetypes registry are clearly new features.  Now if two senior devs like you think otherwise, I’m reconsidering.  These additions can’t possibly break code, can they?  So I can agree with a viewpoint that mimetypes should match what the IANA publishes and that adding missing types is a bugfix.  (It’s less disturbing than updating HTMLParser for example, and I agree with that.)  Georg’s inclusion of a registry addition for IIRC 3.2.2 would also indicate RM support for this viewpoint.

About Antoine’s remark: mimetypes already reads mime.types files, so even if our internal registry is not up-to-date the module should know about all types present in /etc/mime.types.