Comment 15 for bug 231371

r15499 is good. Installer images are copied over from -proposed to -updates exactly as desired. (I haven't explicitly tested dist-upgrader tarballs, but they're very similar to installer images so should be fine too.)

The one flaw I can see is that it copies a little too much - namely, it also copies Rosetta translations, which fail because the copied custom upload doesn't have a .sourcepackagerelease. This is harmless, since we don't actually want or need to copy those translations. I'll prepare another follow-up branch that restricts the copied uploads to only ones in copyable_types.