Currently, Gentoo takes matters into its own hands, as concerns packaging /etc/mime.types, and there are signs that it is atrophying. However, there is a mailcap project, which is maintained by Fedora developers at https://pagure.io/mailcap.
I posit that mailcap should be considered as an upstream, because its commit history demonstrates that it receives updates with greater regularity. Currently, there are five other outstanding bugs against app-misc/mime-types, some of which date from as early as 2015. Three of these - bug 487622, bug 534658 and bug 542190 - would be rectified by switching to mailcap. That leaves another two, which could potentially be resolved by cooperating with the mailcap maintainers.
As a bonus, it includes a script for generating a list of mime types that may be directly included by nginx. Arch Linux takes advantage of this by preventing the installation of the woefully incomplete mime.types file that is included by the nginx sources and, instead, having nginx depend on their mailcap package, which produces /etc/nginx/mime.types. This seems like a sensible approach.
I'm taking the liberty of CC'ing net-mail, on the off-chance that it contains any parties that have any interest in this suggestion.
Another possibility would be to use Debian's mime-types package. They used to have a monolithic mime-support package but elected to turn it into the equivalent of a virtual, with mime-types (currently) providing the /etc/mime.types file. Even the older mime-support package provided by Debian Buster has such issues as bug 719900 covered.