Presumably for now, you need to go into the admin tool, go into the quarantine account, and click View Mail (or share the Quarantine account Inbox with your trusted admins so that they can see it from their own web client)
From there, you should be able to see the quarantined email, save off the affected attachment (assuming your local anti-virus doesn't stop you doing so) then remove the attachment and forward the email to the original intended recipient.
You will have to put the attachment on a network share or somewhere else the recipient can access it.
I haven't tested this method as yet, as I haven't had anything get quarantined since I upgraded - but I assume it should work.
Alternatively, you could use this method that I have been using for the past year on ZCS 6.0.x
Easier Handling of Banned Content and Bad Header Messages