With the milestone Port Media system from v2 we had ported the existing attachment system. The fact is that this system wasn't good enough.
It's always important to start with the original code as that allows us to review past decisions and understand all the corner cases previously considered.
Sadly, what motivated us to re-design the attachment system were the non-considered corner cases. In particular:
titlefrom attachment, as it's part of the relation between attachment and note.
actorfrom attachment, many actors may publish the same attachment.
is_localfrom attachment, as it's part of the relation between attachment and note.
remote_urlfrom attachment, different urls can return the same attachment.
The structural changes and how it compares to v2 is detailed in our wiki.
Some relevant observations:
Linkis now part of GNU social Data Representation terms. It represents any remote URL being shared in a note.