field level descending index?

[bonesoul]

documentation mentions;

  var animalSchema = new Schema({
    name: String,
    type: String,
    tags: { type: [String], index: true } // field level
  });

  animalSchema.index({ name: 1, type: -1 }); // schema level

is it even possible to create a descending index within field level?

tried
tags: { type: [String], index: -1 }

but can't get it working?