And what if you need to reply-to-all AND add a new attachment? I had exactly this problem last night. It is IMPOSSIBLE to do both. You can neither paste the attachment (unless it is a photo) into the reply-to-all message, nor can you copy/paste the reply-to-all address list (50+ addresses) into an email message created in an application with the file. You can copy them individually, but not select all > copy.
Apple: This lack of functionality is a mess!