MDL-42616 filelib: Use config value for custom MIME types (1/2)

Uses a new optional config variable to define custom MIME types
in JSON format. Includes new API for accessing and modifying
this data.

lib/phpunit/classes/util.php

@@ -205,6 +205,8 @@ class phpunit_util extends testing_util {
         core_text::reset_caches();
         get_message_processors(false, true);
         filter_manager::reset_caches();
+        core_filetypes::reset_caches();
+
         // Reset internal users.
         core_user::reset_internal_users();