Display custom RibbonGroup in specific views

Hello,

I would like to display a custom ribbon group only in some views.

For this, I set the ContextByType property to a desired view controller, e.g.:

[RibbonGroup("MyRibbonGroup", ContextByType=typeof(ProjectsController))]

How can I set more than one (e.g. not only Projects, but also Files)?

It must be easy, but I was not able to figure it out.

Thank you