Thanks for the detailed reply!
So basically just pick the "set a different price for this file" option and a current buyer of said bonus file can download whenever. But if the file is gone and replaced with a new one (with a different price even if that helps), then they WILL have to buy the new one. Is it possible to verify this info from somewhere in the faq? Sounds important imo.
If This is the case, that's great, since this means i won't have to worry about emailing bonus files to buyers of the base game, separately.