Enhancement: Make mimetype allow list configurable for app provider

We've added a configuration option to configure the mimetype allow list introduced in cs3org/reva#2095. This also makes it possible to set one application per mime type as a default.

https://github.com/owncloud/ocis/pull/2553
https://github.com/cs3org/reva/pull/2095
https://github.com/owncloud/ocis/issues/2563
