Custom attribute (read-only) in Laravel 5.1

I have Cam model and for each cam I have a folder with images on server. I just need to get all image names in custom attribute but I don't get it. There is no need to call actions within table. I tried extending constructor but no luck.

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Cam extends Model
{
    protected $table = 'cams';
    protected $guarded = 'id';
    public $timestamps = false;
    public $images = null;

    public function __construct($attributes = array())
    {
        parent::__construct($attributes);

        $this->images = 'kk';
    }

    public function getVideosAttribute($value) {
        return explode(',', $value);
    }

    public function city() {
        return $this->belongsTo('App\City');
    }
}

via Chebli Mohamed