openttd.grf is now always loaded and provides all extra graphics in case the (possibly outdated) baseset does not.
orig_extra.grf contains graphics specific to the original baseset only.
only providing the "extra" base graphics in one palette and doing the conversion upon loading; the conversion has existed for a while, but now the NewGRF can tell its palette using Action 14 thus it can enable the conversion for only that NewGRF.