Backport of #8197 to stable6

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 owncloud/security-tracker#56