//Zend\Expressive\Plates\PlatesRenderer
//....
public function getTemplate(){
        if (null === $this->template) {
            $this->template = $this->createTemplate();
        }
        return $this->template;
    }

use League\Plates\Engine;
use League\Plates\Extension\ExtensionInterface;

class ChangeCase implements ExtensionInterface
{
    public function register(Engine $engine)
    {
        $engine->registerFunction('case', [$this, 'getObject']);
    }

    public function getObject()
    {
        return $this;
    }

    public function upper($var)
    {
        return strtoupper($var);
    }

    public function lower($var)
    {
        return strtolower($var);
    }
}

//template.global.php
'templates' => [
        'extension'         => 'phtml',
        'paths'             => [
            'app'    => ['templates/app'],
            'layout' => ['templates/layout'],
            'error'  => ['templates/error'],
            'user'   => ['templates/user'],
        ],
        'plates_extensions' => [
            Common\PlatesExtension\ChangeCase::class,
        ],
    ],

use Interop\Container\ContainerInterface;
use Zend\Expressive\Template\TemplateRendererInterface;

class LoadPlatesExtensionFactory
{

    public function __invoke(ContainerInterface $container)
    {

        $template = $container->get(TemplateRendererInterface::class);
        $config = $container->get('config') ? $container->get('config') : [];
        $platesExtensions = [];
        if (isset($config['templates']['plates_extensions'])) {            
                $platesExtensions = $config['templates']['plates_extensions'];            
        }
        return new LoadPlatesExtension($template, $platesExtensions);
    }

}

use League\Plates\Extension\ExtensionInterface;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Zend\Expressive\Template\TemplateRendererInterface;

class LoadPlatesExtension
{

    protected $platesEngine;
    protected $platesExtensions;

    public function __construct(TemplateRendererInterface $platesEngine, array $platesExtensions = [])
    {
        $this->platesEngine = $platesEngine;
        $this->platesExtensions = $platesExtensions;
    }

    public function __invoke(ServerRequestInterface $request, ResponseInterface $response, callable $next  = null)
    {
        $this->_loadExtensions();
        return $next($request, $response);
    }

    private function _loadExtensions(){
        if(count($this->platesExtensions)>0){
            foreach($this->platesExtensions as $extenstion) {
                if($extenstion instanceof ExtensionInterface)
                    $this->platesEngine->loadExtension( new $extenstion() );
                    //after editing PlatesRenderer class it works
                   //$this->platesEngine->getTemplate()->loadExtension(new $extension());
                else
                    throw \Exception(sprintf("%s not implement %s", $extenstion, ExtensionInterface::class));
            }
        }
    }
}

use League\Plates\Extension as PlatesExtension;

// Create the engine instance:
$engine = new PlatesEngine();

// Enable asset extension
$engine->loadExtension(new PlatesExtension\Asset($config['paths']['assets']));

<img src="<?=$this->asset('zf-logo.png')?>" alt="Zend Expressive" />

    'templates' => [
        'extension' => 'phtml',
        'paths' => [
            'assets' => 'public',
            'app'    => ['templates/app'],
            'layout' => ['templates/layout'],
            'error'  => ['templates/error'],