Attachments: might be a really daft question, but: Attachments are not available in LEX - it is recommended to move, e.g. using Magic Mover app.

However: why do we need to move Attachments at all? Looking at Lightning, the Attachments list is still there and all attachments can be viewed. When you upload new attachment it uploads as file.

So why move at all? We have over 91'000 attachments, most on them on email messages from email to case. For these attachments, the Magic Mover app doesn't work anyway.