Author: Dan Poltawski
MDL-32134 lang: Add missing PNG string
http://git.moodle.org/gw?p=moodle.git;a=commit;h=f7b16c4220b94b468b0f1414d7af7b2e822ee441
http://github.com/moodle/moodle/commit/f7b16c4220b94b468b0f1414d7af7b2e822ee441
+ 2.3 en [image/png,core_mimetypes]