White-list known secure mime types. Refs. #8184

This extends mimetypes.list.php to be a white-list of known secure mime types as well as offering secure alternatives for know potentially malicious mime types.
I kept this in the `OC_Helper` and `OC\Files\Type\Detection` classes as each backend has its own way of detecting mime types.

This supersedes #8184 as discussed in https://github.com/owncloud/security-tracker/issues/56

@LukasReschke @karlitschek @icewind1991