If you check the MSDN definition of ActionFilterAttribute you will see that it is an abstract class that inherits interfaces IActionFilter AND IResultFilter.
In effect inheriting from ActionFilterAttribute,
public class CustomFilter : ActionFilterAttribute
is equivalent to inheriting class FilterAttribute and interfaces IActionFilter and IResultFilter
public class CustomFilter : FilterAttribute, IActionFilter, IResultFilter
and is no different than
public class CustomFilter : ActionFilterAttribute, IActionFilter, IResultFilter
So inheriting from ActionFilterAttribute is no different than inheriting from ActionFilterAttribute and IActionFilter.