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Overall, goodies in gog follow a pattern, "game_name_goodie.zip". But there are some random exceptions (e.g. Blood 2 manual is not zipped, DUSK goodies are just "goodie_name.zip", etc.).

If one of them receives some sort of update (which is rare, but happens in newer products, like when a soundtrack for an in dev game is updated with new tracks) it is really easy to miss. I noticed this after finding out the DUSK HD extra received some updates fixing problems.
The easiest way to be sure is to check gogdb for updates.

There is other drm-free store, zoom-plataform, where the goodies follow a naming standard and version number in the end - and that is very helpful to keep your collection properly organized and updated - So I am going to ask for this to be a feature for gog.

1. Add a version number for goodies files (e.g. doom_manual_v1.zip);
2. Stricter naming standard (don't allow files to be named just "manual.zip", etc.).