Currently, the kernel supports UTF-8 up to version 12.1. mkfs will always choose the latest version, but attempting to run a filesystem with a more recent UTF-8 version than the kernel supports is risky, and to preserve your data, the kernel will refuse to mount such filesystem. These applications will fail if the kernel throws an error on what they expect is a valid name, so by default, if an application tries to use an invalid name on a case-insensitive directory, the kernel will just let it happen, and treat that single file as an opaque byte sequence. After mounting a case-insensitive enabled filesystem, it is now possible to flip the 'Casefold' inode attribute ('+F') in empty directories to make the lookup of files inside them case-insensitive: