Ah good issue to raise @angus might want to relax that limit
It’s the standard out of the box Discourse preview layout for internal and external links. Similar to the preview Facebook generates (but focuses less on highlighting the images). So instead of having just an ugly URL the user is presented with a nicely formatted preview/excerpt box which gives them a better idea of what the link links to.
Unlike Facebook and because Discourse is a forum not a microblogging system there’s a Topic view (where posts live) and a Topic List view. Oneboxes live in posts. The plugin presents thumbnails on the Topic List.