Remarkup - call pasted files "pasted_file" rather than "undefined"

Summary: Fixes T5917. Turns out that pasted files don't get a name - "what is data on a clipboard anyway?" / see https://code.google.com/p/chromium/issues/detail?id=361145

Test Plan: pasted a file and got the slightly better name "pasted_file"

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5917

Differential Revision: https://secure.phabricator.com/D10310