Fix bad aspect ratio on some file previews

Summary: Ref T7707. Fixes T4724. I misread the report on T4724; this is trivial. We're just reading the wrong properties in setting "width" and "height" attributes, the actual thumbnailing logic is fine.

Test Plan: Uploaded image from T4724, saw it have a proper aspect ratio.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: spicyj, epriestley

Maniphest Tasks: T4724, T7707

Differential Revision: https://secure.phabricator.com/D12810