How to play .caf file

I tried to change my code from

let urlString = Bundle.main.path(forResource: "X", ofType: "mp3")!

to

let urlString = Bundle.main.path(forResource: "X", ofType: "caf")!

after I already uploaded the file. But it throws a fatal error: Unexpectedly found nil while unwrapping an Optional value

I double check the filename and it's correct. and the caf file does play.