Add a configuration switch for enabled preview mimetypes

Backport of https://github.com/owncloud/core/pull/11211 to stable6
Merge branch 'stable6' into backport-11211-stable6

Conflicts:
	config/config.sample.php
Merge pull request #11246 from owncloud/backport-11211-stable6

Add a configuration switch for enabled preview mimetypes