The accepted extension was `.mpskin.json`, but now the app looks for .json, .txt, and .mpskin.json, in that order.
All the default skins' config files are changed to `.json`, but any custom skins made before this update having `.mpskin.json` will still work. Wiki and a template for sharing a custom skin on GitHub also got updated to follow the change. Now you can directly load a custom skin downloaded from the Discussions category.
As the result, the initial loading impact has gotten bigger while fetching the config files in all accepted extensions. If it's found out to be troublesome, I may have to revert some of the changes and only support `.json`. Unless I can figure out how to replace a recursion holding a Promise with an iteration.