FilterTestAssets.php

<?php

namespace Drupal\filter_test\Plugin\Filter;

use Drupal\filter\FilterProcessResult;
use Drupal\filter\Plugin\FilterBase;

/**
 * Provides a test filter to attach assets.
 *
 * @Filter(
 *   id = "filter_test_assets",
 *   title = @Translation("Testing filter"),
 *   description = @Translation("Does not change content; attaches assets."),
 *   type = Drupal\filter\Plugin\FilterInterface::TYPE_TRANSFORM_REVERSIBLE
 * )
 */
class FilterTestAssets extends FilterBase {
    
    /**
     * {@inheritdoc}
     */
    public function process($text, $langcode) {
        $result = new FilterProcessResult($text);
        $result->addAttachments([
            'library' => [
                'filter/caption',
            ],
        ]);
        return $result;
    }

}