(Celerity) Fix mimetype for JPEG files so IE will load them

Summary: IE won't load background images in a page that are served with the mimetype "image/jpg" as it only recognises the "image/jpeg" mimetype.

Test Plan: Spent an hour or two going back and forth between Linux (to dev) and Windows (to test) to find the source of this issue, then flipped several tables at IE for being terrible.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8689