Both upload methods should work.
There has been problems with code "fi-fi" not showing up. If you do not see code "fi-fi" when going to Home -> Languages -> Edit or Add language, there is usually only one way to fix this.
Solution is reinstall Opencart 3.X.
Before you do reinstall Opencart 3.X, please clear cache and if you use Cloudflare, turn on "Development Mode" in Cloudflare dashboard.