FileField.php

<?php

namespace Drupal\file\Plugin\migrate\field\d7;

use Drupal\file\Plugin\migrate\field\d6\FileField as D6FileField;
use Drupal\migrate\Plugin\MigrationInterface;

/**
 * @MigrateField(
 *   id = "file",
 *   core = {7},
 *   source_module = "file",
 *   destination_module = "file"
 * )
 */
class FileField extends D6FileField {
    
    /**
     * {@inheritdoc}
     */
    public function defineValueProcessPipeline(MigrationInterface $migration, $field_name, $data) {
        $process = [
            'plugin' => 'sub_process',
            'source' => $field_name,
            'process' => [
                'target_id' => 'fid',
                'display' => 'display',
                'description' => 'description',
            ],
        ];
        $migration->mergeProcessOfProperty($field_name, $process);
    }

}