TableFormatter.php

<?php

namespace Drupal\file\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;

/**
 * Plugin implementation of the 'file_table' formatter.
 *
 * @FieldFormatter(
 *   id = "file_table",
 *   label = @Translation("Table of files"),
 *   field_types = {
 *     "file"
 *   }
 * )
 */
class TableFormatter extends DescriptionAwareFileFormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        if ($files = $this->getEntitiesToView($items, $langcode)) {
            $header = [
                t('Attachment'),
                t('Size'),
            ];
            $rows = [];
            foreach ($files as $delta => $file) {
                $item = $file->_referringItem;
                $rows[] = [
                    [
                        'data' => [
                            '#theme' => 'file_link',
                            '#file' => $file,
                            '#description' => $this->getSetting('use_description_as_link_text') ? $item->description : NULL,
                            '#cache' => [
                                'tags' => $file->getCacheTags(),
                            ],
                        ],
                    ],
                    [
                        'data' => format_size($file->getSize()),
                    ],
                ];
            }
            $elements[0] = [];
            if (!empty($rows)) {
                $elements[0] = [
                    '#theme' => 'table__file_formatter_table',
                    '#header' => $header,
                    '#rows' => $rows,
                ];
            }
        }
        return $elements;
    }

}