<?php
add_action('rest_insert_attachment', function($attachment, $request, $creating) {
  $data = $request->get_params(); // Extra data that was sent to /wp/v2/media
  // In this example we can use $data['entry_id']
  // Something like: update_post_meta((int) $data['entry_id'], 'attachment_id', $attachment['ID']);
});