It's been pointed out to me that I neglected to merge the "intent to
prototype" requirements into my email. So:
Platform coverage: everywhere.
Preference: no pref.
DevTools bug: covered by existing network tooling (it already shows the
Other browsers: as noted, they already do this
web-platform-tests: TBC. We have internal tests which were updated, but
it doesn't appear WPT is set up to deal with this right now (by their
nature, "attachment" content doesn't load in-browser) as it'd need to
handle downloads and provide some way of asserting things about them.
This will likely be tackled after / as part of spec changes.