Maybe a stupid question, how to create a "cmf:image" and "cmf:file" mixin? I thought I understood what [mixins](http://jackrabbit.apache.org/node-types.html) are, however I cannot figure out how to define one, so apparently I do not. I checked the PHPCR-ODM documentation, and searched code and tests for examples and definitions. What is actually a mixin in PHPCR-ODM Document context? Is it similar as using a trait containing properties and methods and/or more like the doctrine extensions?