Play MP4 in ExoPlayer the simplest way

I'm new to Android Development here, and I'm having a hard time integrating and using ExoPlayer in my project. I found a lot of tutorials but as expected I am getting a lot of different approaches and most are outdated and hard to build. I found one approach that works, but the problem is, that it plays m3u8 files. I want to make Exoplayer play mp4 files (from a URL). Here is the code for the Exoplayer demo that works for me but only plays m3u8 files.

private lateinit var binding: ActivityMainBinding
private var exoPlayer: ExoPlayer? = null
private var playbackPosition = 0L
private var playWhenReady = true

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    window.setFlags(
        WindowManager.LayoutParams.FLAG_FULLSCREEN,
        WindowManager.LayoutParams.FLAG_FULLSCREEN
    )
    binding = ActivityMainBinding.inflate(layoutInflater)
    val view = binding.root
    setContentView(view)
    preparePlayer()
}

private fun preparePlayer() {
    exoPlayer = ExoPlayer.Builder(this).build()
    exoPlayer?.playWhenReady = true
    binding.playerView.player = exoPlayer
    val defaultHttpDataSourceFactory = DefaultHttpDataSource.Factory()
    val mediaItem =
        MediaItem.fromUri(URL)
    val mediaSource =
        HlsMediaSource.Factory(defaultHttpDataSourceFactory).createMediaSource(mediaItem)
    exoPlayer?.setMediaSource(mediaSource)
    exoPlayer?.seekTo(playbackPosition)
    exoPlayer?.playWhenReady = playWhenReady
    exoPlayer?.prepare()
}

private fun releasePlayer() {
    exoPlayer?.let { player ->
        playbackPosition = player.currentPosition
        playWhenReady = player.playWhenReady
        player.release()
        exoPlayer = null
    }
}

override fun onStop() {
    super.onStop()
    releasePlayer()
}

override fun onPause() {
    super.onPause()
    releasePlayer()
}

override fun onDestroy() {
    super.onDestroy()
    releasePlayer()
}

companion object {
    const val URL =
        "sample m3u8 file URL here"
}

Please help me tweak or make the code play mp4 files. Please bare with me I'm totally new to Android development and ExoPlayer. I will greatly appreciate your help.