Through jAlbum 37, this routine, in earlyinit.groovy, properly excluded unsupported object types from my skins:

CategoryFilter categoryFilter = new CategoryFilter().include(Category.folder, Category.image, Category.video, Category.webLocation).include("pdf");
rejectCounter = new RejectCounterFilter(categoryFilter);
factory.setFilter(rejectCounter);

That was followed by a warning to the user triggered in init.groovy:

if (rejectCounter.getTotalCount() > 0) {
	System.out.println("Skipped types: " + rejectCounter);
	engine.fireWarning("In Neptune, only folders, images, videos, web locations, and PDF's are supported. Other objects have been skipped.");
}

Starting with jAlbum 38 and continuing in jAlbum 39, this no longer has any effect at all, and the skin tries to deal with things like Word documents, resulting in broken albums.

Fixed in https://jalbum.net/download/beta/jalbum-core.jar

I had to move the earlyinit call even earlier. With v38, it was put just after makeViews(), and makeViews() also triggered building the entire album object tree, before the filters were applied.

So, the effect of this change is that you won't have access to views within earlyinit.

Another fix is that the ThemeImageProcessor now also handles detecting changes to AVIF theme images, and not just with JPEGs.

Yes, that's actually a good assumption