Attachment Taxonomies is a WordPress Core feature plugin to improve support for attachment taxonomies and possibly ship a default one.
The current level of support for attachment taxonomies is rudimentary. They can easily be registered, but won't show up in a proper UI as part of the media library.
The feature plugin is originally based on the plugin Attachment Taxonomies, but will need an overhaul and several improvements to possibly become part of Core.
attachment-taxonomies-feature
folder to the /wp-content/plugins/
directory.