Thanks a lot for your answers and examples @cnbrownlie and @gitdemont ! I have many things to try then.
I also found a way to circumvent the issue, but if feels a bit hacky, I added this line : addResourcePath(prefix = "www", directoryPath = "./www"), but it does not work for every resource I think.This seems to work in Chromium for all the resources, but not really in Firefox for videos (I posted an issue about it, the behavior is weird). Edit : this seems unrelated.