Use "Best" URI when linking to files from Maniphest file previews

Summary: Previously, this code accidentally did not use the best URI. Instead,
use the best URI. It's the best, obviously.

Test Plan: Uploaded a binary file and then clicked the preview.

Reviewers: hunterbridges, jungejason, nh, tuomaspelkonen, aran

Reviewed By: tuomaspelkonen

CC: aran, tuomaspelkonen

Differential Revision: 905