That sounds like exactly the right thing to do. So we need to add stuff
here, without taking anything away.
That would be a shame: there are too many things in Emacs that just get more complicated over time. I'm dubious about the utility of system-independent configuration here too.
Looking even more closely at this, I guess `xdg-open' could be used as
some final fallback, but it doesn't seem to add anything that isn't
already available from mailcap?
xdg-open uses the underlying desktop system's MIME infrastructure. For a GNU system, that would most typically be GNOME, and its "gio open" (or on older systems, gvfs-open) command.
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