I might change this convention again in the future, particularly once
someone wants to fund C89 conformance, but let's stay consistent for
now.
Part of P0264, funded by [Anonymous] and Blue Bolt.
We've been using the three-letter extension names for everything else
that isn't .DAT, so it's more consistent to do the same for .MPN,
however particularly nice and readable its FOURCC may be. 8.3 filenames
also really appreciate that they'll have to accommodate one fewer
letter.
Part of P0138, funded by [Anonymous] and Blue Bolt.