Fix displaying of raw files in Differential

Summary:
See D1533#5.
Also deduplicates logic of what is stored to blob in ArcanistDiffWorkflow.

Blame Rev: D1533

Test Plan:
Display raw version of text file.
Display raw version of image.

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1583