At least for audio output to a Bluetooth receiver, I can confirm it works.

Regarding Bluetooth commands, I am not sure, as I don't have any smart Bluetooth device to pair it with. If it does not work out of the box, maybe this could help.

For file transfers with an official Jolla C, no success. Obviously it should be possible because this exists, but to be fair I never managed to transfer any file with Bluetooth in SFOS, even on official SFOS releases.