Sorry for the late notice, but I just realized a
couple of things wrong with how issue #32256 is
implemented - hopefully not hard to fix. As it
stands, the impl (and I guess the API) forces you
to use GIFs for icons, or at least images named
*.gif. But GIF is sort of a deprecated image
format; PNG is better for the future, generally.
We should definitely ensure that everything
supports PNGs.

SystemAction.iconResource takes a full image path
with extension so it is clean. But the 'iconBase'
attr on Action does not. IMHO it should include
the extension, and when computing variants, insert
their suffixes before the last '.' (as is normal
for localized & branded variants of resources, too
- see e.g. nbres URL protocol).

See issue #32102 for more background. Actions,
like SystemAction, is already using the variant of
Utilities.loadImage that autolocalizes the image,
so this is not a problem (I think).

Currently it seems that a SystemAction specifying
an iconResource of foo/bar.png will wind up with
*no* icon, as Actions will try to load
foo/bar.gif. This is definitely a bug (regression).

Also it seems that Actions.updateButtonIcon first
calls button.setIcon with a SMALL_ICON, *then*
checks for iconBase and possibly calls setIcon
again - you should avoid calling setIcon twice.
This will be the normal case since SystemAction
would (I guess) return non-null for both keys.
Decide which one takes precedence and use it.