From the Pathfinder Core Rulebook (page 154):
Mithral is a very rare silvery, glistening metal that is lighter than steel but just as hard. When worked like steel, it becomes a wonderful material from which to create armor, and is occasionally used for other items as well. Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor. A character wearing mithral full plate must be proficient in wearing heavy armor to avoid adding the armor's check penalty to all his attack rolls and skill checks that involve moving.
When creating Mithral Full Plate armor, my Ranger (light/medium armor only) is proficient with it and should not be.
Datasets loaded: Pathfinder core rules, Pathfinder Traits, Pathfinder Beastiary