Easy. Add platform behavior to it, tick solid. Everything that a behavior requires should be provided by the behavior itself.
That way you could have moving platforms etc., sprites that act as solid ground to walk onto (viking with up shield) etc
What if someone wanted an object to not become solid at some time? What if he wanted to make the object solid for some, but not solid for others (ghosts)? Hardcoded attributes would be a bad idea, as they wouldn't be as flexible as a behavior.