Don't consider file references inside quoted text blocks to grant "attachment intent"

Summary:
Ref T13682. This is a marginal case, but if you quote someone else's text and it has a file reference embedded in it, that isn't good enough to generate "attachment intent".

Since you need both a reference and an explicit attachment, this should never actually affect any user-visible behavior today, but makes the ruleset more thorough.

Test Plan: Dragged and dropped a file, referencing it in either a quoted or unquoted block. Saw it attach only for a quoted block.

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21833