[ticket/12541] Activate attachments tab when files are dropped into textarea

PHPBB3-12541
Merge remote-tracking branch 'PayBas/ticket/12541' into develop-ascraeus

* PayBas/ticket/12541:
  [ticket/12541] Activate attachments tab when files are dropped into textarea

Conflicts:
	phpBB/styles/prosilver/template/forum_fn.js