The ingredients in the Medication resource are intended to be a list of substances that are to be combined in a compound or extemporaneous preparation.  To the end, we believe that knowing the active ingredients are important for clinical decision support systems but do not believe that knowing the type of inactive ingredients is important enough to expose in the core resource.  It can be done in an extension and we would point you to the MedicationKnowledge resource (MedicationKnowledge.definitional.ingredient.type) to see how we have modelled it there.