MimeTypeController generates a map of file extensions to MIME types

It uses `mime.types` file from apache http located under
http://svn.apache.org/viewvc/httpd/httpd/trunk/docs/conf/mime.types?view=markup

This file has been placed in the public domain for unlimited
redistribution,
so we can use it and ship it with Yii.

see also yiisoft/yii#2851 and yiisoft/yii#2857