title: Laravel Storage::extend not working
body: <p>I can't figure out where I'm going wrong, with this. I've followed the Laravel docs by installing <code>spatie/flysystem-dropbox</code> via composer copied the DropboxServiceProvider from the Laravel Docs, added the service provided to the <code>config\app.php</code>, ran <code>composer dump autoload</code> but yet I am still getting the following error message:</p>
  
  <pre><code>PHP error:  Undefined index: driver in /***/vendor/laravel/framework/src/Illuminate/Filesystem/FilesystemManager.php on line 112
  </code></pre>
  
  <p>Here is the service provider:</p>
  
  <pre><code>&lt;?php
  
  namespace App\Providers;
  
  use Storage;
  use League\Flysystem\Filesystem;
  use Spatie\Dropbox\Client as DropboxClient;
  use Illuminate\Support\ServiceProvider;
  use Spatie\FlysystemDropbox\DropboxAdapter;
  
  class DropboxServiceProvider extends ServiceProvider
  {
      /**
       * Perform post-registration booting of services.
       *
       * @return void
       */
      public function boot()
      {
          Storage::extend('dropbox', function ($app, $config) {
              $client = new DropboxClient(
                  $config['authorizationToken']
              );
  
              return new Filesystem(new DropboxAdapter($client));
          });
      }
  
      /**
       * Register bindings in the container.
       *
       * @return void
       */
      public function register()
      {
          //
      }
  }
  </code></pre>
  
  <p>And here's my config/app/php:</p>
  
  <pre><code>...
          /*
           * Application Service Providers...
           */
          App\Providers\AppServiceProvider::class,
          App\Providers\AuthServiceProvider::class,
          App\Providers\DropboxServiceProvider::class,
          // App\Providers\BroadcastServiceProvider::class,
          App\Providers\EventServiceProvider::class,
          App\Providers\RouteServiceProvider::class,
  
  ...
  </code></pre>
  
  <p>Finally, here's my config/filesystems.php:</p>
  
  <pre><code>'dropbox'=&gt;[
              'authorizationToken'=&gt;env('DROPBOX_ACCESS_TOKEN')
          ],
  </code></pre>