Integration with Attachments Plugin

I figured it out

Add the code below to functions.php

function attachments( $attachments ) {
  $args = array(
    'post_type'     => array( 'post','page','event' ), // Adds event as a post type that supports attachments
  );

  $attachments->register( 'attachments', $args ); // unique instance name
}

add_action( 'attachments_register', 'attachments' );