Added PNG extension to appease EXImageLoader

Summary: Append PNG path extension to appease EXImageLoader. Note that our code ignores file extensions and looks at the magic numbers directly, and shouldn't be affected by this change.

Test Plan: NA

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D609