[{"_id":"grandParentId","types":[{"_id":"parentId","files":[{"url":"1.example.com","_id":"1childId"},{"url":"2.example.com","_id":"2childId"}]}]}]

db.collection.aggregate([{"$unwind":"$types"},{"$unwind":"$types.files"},{"$match":{"types.files._id":"2childId"}},{"$project":{"url":"$types.files.url","_id":"$types.files._id"}}])