This adds a JavaScript hook for extending the File Attachment thumbnail

container UI. The hook can be used to display additional content on

file attachment thumbnails and in the file attachment actions menu.
This also adds a method to the File Attachment thumbnail view that allows

callers to add an action to the file attachment actions menu.

Ran JS unit tests.
Ran unit tests.
Manually tested with the new "Download as PDF" action that's being added in

  in Power Pack.
Manually tested with a simple extension and view, the one in the example

  in the docs.
Built the docs and made sure they look ok.