[Files.app] Move the unknown-file-handling code to file_tasks.js

This is a refactoring patch. No functionality is changed.

BUG=none
TEST=manually testted

Review URL: https://codereview.chromium.org/47933002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231307 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231307