What's the correct extension for opus file for AVURLAsset?

Question

I have an opus audio file I recorded as follows:

AVAudioRecorder(url: url, settings: [
  AVFormatIDKey: Int(kAudioFormatOpus),
  AVSampleRateKey: 24000,
  AVNumberOfChannelsKey: 1
])

I can play the file without problem with an AVAudioPlayer. Now I'm trying to show a waveform for it, and I think for this purpose I need to load it into an AVAsset. My code is as follows:

let asset = AVURLAsset(url: audioURL, options: [AVURLAssetPreferPreciseDurationAndTimingKey: NSNumber(value: true as Bool)])

asset.loadValuesAsynchronously(forKeys: ["tracks", "duration"]) {
  var error: NSError?

  var loadedTrack: AVAssetTrack?
  switch asset.statusOfValue(forKey: "tracks", error: &error) {
  case .loaded:
    loadedTrack = asset.tracks.first
  case .failed, .cancelled, .loading, .unknown:
    log.error("Couldn't load asset for tracks: \(error?.localizedDescription ?? "Unknown error")")
    break
  default:
    break
  }

  ...
}

This unfortunately doesn't work and gives me the following error:

Couldn't load asset for tracks: Cannot Open

My URL is a local URL to the asset with an extension of .opus. The file exists and can be played with the AVAudioPlayer aforementioned. I read in a few SO posts that AVURLAsset requires file extension to work, and I'm wondering if .opus isn't the right one. If that's the case, what's the correct extension?

If that's not the cause, would love to know how else I can fix my code here. Thank you!