EAttachmentHandler predates EExtension, so this is really just a code
cleanup to use the extension framework. But this also demonstrates that
extensions can target interfaces as well as instantiable types, since
EAttachmentView is an interface.
What's also nice is EAttachmentView no longer has to directly interact
with attachment handlers. Instead of EAttachmentView having to query
each attachment handler for drag-and-drop info, each handler now pushes
its own drag-and-drop info to its EAttachmentView during initialization.
8223eaf570