BUG: fix a bug where PlotContainer.save would wrongfully ignore specified field extension

[neutrinoceros]

PR Summary

fix #3677, see discussion therein

[neutrinoceros]

switching to draft as I add/fix new tests

[neutrinoceros]

Something terrible happened in image tests and I got a conflict + typing errors (most of them are not easily fixable, it seems, so I just went with type: ignore...)

[neutrinoceros]

@yt-fido test this please

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout yt-4.0.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -m1 ad8999239ef64122ba1c9ff9a7ad8343130d4595

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #3687: BUG: fix a bug where PlotContainer.save would wrongfully ignore specified field extension'

4. Push to a named branch:

git push YOURFORK yt-4.0.x:auto-backport-of-pr-3687-on-yt-4.0.x

5. Create a PR against branch yt-4.0.x, I would have named this PR:

"Backport PR #3687 on branch yt-4.0.x (BUG: fix a bug where PlotContainer.save would wrongfully ignore specified field extension)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.

[neutrinoceros]

The bug (#3677) is still present on the yt-4.0.x branch so I'll open a manual backport