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