exchange/app/Http/Controllers/Api/MultimediaController.php

<?php

namespace App\Http\Controllers\Api;

use App\Models\MultimediaCategory;
use App\Transformers\MediaTransformer;

class MultimediaController extends ApiController
{
    public function index($category)
    {
        $medias = MultimediaCategory::where('id', $category)->with('medias')->get()->first();
        if($medias){
            return $this->respondWithCollection($medias->medias, new MediaTransformer());
        }
        return $this->errorNotFound();
    }
}