CybOX Relationships (as reference) - File Contains Another File

example.json

{
    "type": "package",
    "spec_version": "stix-2.0",
    "observations": [{
        "type": "observation",
        "id": "observation--4",
        "spec_version": "stix-2.0",
        "created_at": "2016-03-21T01:01:01Z",
        "observed_at": "2016-03-21T01:01:01Z",
        "cybox": {"objects": [
            {
                "type": "file-object",
                "id": "file-object--1",
                "hashes": {"md5": "66e2ea40dc71d5ba701574ea215a81f1"},
                "size": 641028,
                "contains_ref":["file-object--2"]
            },
            {
                "type": "file-object",
                "id": "file-object--2",
                "hashes": {"md5": "3E4356B9C1BC08AEE869F37CD86359B4"},
                "size": 25536
            }
        ]}
    }]
}