Author: Dan Poltawski
MDL-32134 lang: Add missing PNG string
http://git.moodle.org/gw?p=moodle.git;a=commit;h=e44909684243be8d3c3b70a4db0f74f55b629bdb
http://github.com/moodle/moodle/commit/e44909684243be8d3c3b70a4db0f74f55b629bdb
+ 2.2 en [image/png,core_mimetypes]