Author: Ruslan Kabalin
MDL-63453 filetypes: Introduce media_source mimetype group.
This is required to group together media streaming techniques that are
handled by Media Source Extensions (MSE) in the browser.
https://developer.mozilla.org/en-US/docs/Web/API/Media_Source_Extensions_API
http://git.moodle.org/gw?p=moodle.git;a=commit;h=037124bf1c99fdc4c4473d3bf0acb2b314e9cc6d
http://github.com/moodle/moodle/commit/037124bf1c99fdc4c4473d3bf0acb2b314e9cc6d
+ 3.8 en [application/dash_xml,core_mimetypes]
+ 3.8 en [application/x-mpegURL,core_mimetypes]
+ 3.8 en [group:media_source,core_mimetypes]