Allow Diffusion to display PDF files

Summary:
When Diffusion encounters an image file, it displays it as an
image, but when it encounters a PDF file, it currently shows only some
gibberish. This fixes that.

Test Plan:
I tried it. Embedding a large PDF in a data URL is a little
bit slow, but it works.

Reviewers: tuomaspelkonen, epriestley, gc3, waltermundt, jungejason, nh

Reviewed By: epriestley

CC: aran, tuomaspelkonen, epriestley, jaapweel

Differential Revision: 915