Per https://apple.stackexchange.com/a/9883/206073 , default file opening by content type should be set via
LSHandlers -> LSHandlerContentType in
defaults write com.apple.LaunchServices/com.apple.launchservices.secure.plist LSHandlers -array[-add]). The format of
LSHandlerContentType is in what is apparently the Mac's way of indicating MIME types, e.g., "public.css". (And I know that
defaults read /System/Library/CoreServices/CoreTypes.bundle/Contents/Info.plist can show how the Mac associates these content types to MIME types (and also how
UTTypeConformsTo there will indicate a hierarchy between types).)
However, according to https://superuser.com/a/421865/156958 , content type can also be associated by setting
LSHandlers -> LSHandlerContentTag with a MIME type (e.g., "text/css") and setting
LSHandlerContentTagClass to "public.mime-type". I don't see any such settings in my own
defaults read com.apple.LaunchServices/com.apple.launchservices.secure.plist LSHandlers, however.
I am unclear on which approach (
LSHandlerContentTagClass of "public.mime-type")) is to be preferred (or which gets preference), or if there is any reason to use both.
As a bonus, I'd also be interested to know how
UTExportedTypeDeclarations may fit into this (e.g., if these indications of third-party MIME types affects which of the two formats to use).