Download .wav files

I have .wav files that I want to download. I'm trying to download them but it does not fit, it came out downloading .mp3 but not .wav.

I share my code that I have:

  

Button that triggers the controller method and passed it as a parameter   the name of the file (in-171-115-20180502-163053.wav).

<a type="button" class="btn btn-primary" href="<?=base_url()?>index.php/login_controller/download_audio/<?php echo($archivowav);?>">
<i class="fa fa-download"></i> Descargar
</a>

  

Function that performs the download of the file.

public function download_audio($audio) {
  $path = file_get_contents(base_url()."assets/grabaciones/".$audio); // get file name
  force_download($audio, $path); // start download'
}

  

This is the error that marks me.