Default Application selector offers only "Web", even though Chromium is installed

I'm having trouble setting the default/preferred browser to Chromium.

Chromium is installed. Chromium thinks it is the default browser. xdg-mime thinks Chromium is the default browser.

Gnome Control Center shows "Web" as the default browser. Gnome Control Center's selector doesn't offer Chromium as an option. When I press the /org/gnome/settings-daemon/plugins/media-keys/www keyboard shortcut, Web (epiphany) is launched. I would like this to launch Chromium. Gnome Control Center's "application" page for Chromium doesn't contain the "default handler" control described in #450.

$ xdg-mime query default x-scheme-handler/http
chromium-browser.desktop

Note: This is on NixOS, which sometimes puts files in different places than normal.