Usually in unit testing, behaviour refers to externally observable behaviour.
Using your example the observable behaviour would be the list of components that you get back from get. The fact that they come from a file is implementation, so I would advise building your tests around the list of components you get back without mocking the file retrieval as it is internal to the class, with setup code to provide an appropriate file.
The alternative would be to make the file which the components were loaded from a dependency for the class, e.g. make it a constructor parameter or method parameter, to allow the file to be specified externally to the class. In this case it would be external and so I would mock it to ensure you got consistent behaviour from it to ensure you're class was using it correctly.