Add extra field to a default rest API endpoint

<?php

/**
 * REST API Fields
 */
class RestFields {
	/**
	 * Construct
	 */
	public function __construct() {
		// REST API fields
		add_action( 'rest_api_init', array( $this, 'register_rest_fields' ) );
	}

	/**
	 * Register rest fields
	 */
	public function register_rest_fields() {
		register_rest_field(
			'download',
			'download_fields',
			array(
				'get_callback' => array( $this, 'get_download_fields' ),
			)
		);
	}

	/**
	 * Download fields
	 *
	 * @return array Download fields
	 */
	public function get_download_fields( $object, $field_name, $request ) {
		$id = $object['id'];

		$values = array(
			'download_category' => get_field( 'download_category', $object['id'] ),
			'download_file_url' => get_field( 'download_file_url', $object['id'] ),
		);

		return $values;
	}
}