Upload audio error pada codeigniter

Saya ingin mengupload file audio ke server dengan menggunakan codeigniter, file yang diijinkan untuk diupload hanya boleh berekstensi mp3 dan m4a. Tapi saat upload muncul error : the filetype you are attempting to upload is not allowed.

public function _do_upload_msc($name){
    $config['upload_path']          = DEFAULT_UPLOAD_MSC;
    $config['allowed_types']        = 'm4a|mp3';
    $config['max_size']             = 50000;
    $config['file_name']            = $name;

      $this->load->library('upload', $config);
      if(!$this->upload->do_upload('file')) //upload and validate
        {
            $data['inputerror'][] = 'file';
      	    $data['error_string'][] = 'Upload error: '.$this->upload->display_errors('',''); //show ajax error
      	    $data['status'] = FALSE;
      	    echo json_encode($data);
      	    exit();
	 }
	return $this->upload->data('file_name');
  }

saat saya menggunakan var_dump hasil arraynya seperti ini:

array (size=5)
      'name' => string 'See U Later.m4a' (length=15)
      'type' => string 'audio/x-m4a' (length=11)
      'tmp_name' => string 'C:\Program Files (x86)\Ampps\tmp\php20AC.tmp' (length=44)
      'error' => int 0
      'size' => int 2425205

kalau pakai print_r :

Array
(
    [file_name] => See U Later.m4a
    [file_type] => video/mp4
    [file_path] => C:/Program Files (x86)/Ampps/www/upload/music/
    [full_path] => C:/Program Files (x86)/Ampps/www/upload/music/See U Later.m4a
    [raw_name] => See U Later
    [orig_name] =>
    [client_name] => See U Later.m4a
    [file_ext] => .m4a
    [file_size] => 2425205
    [is_image] =>
    [image_width] =>
    [image_height] =>
    [image_type] =>
    [image_size_str] =>
)

Kira-kira apa masalahnya ya? terimakasih.