mongo-python-driver/test/client-side-encryption/limits/limits-encryptedFields.json

{
    "fields": [
        {
            "keyId": {
                "$binary": {
                    "base64": "LOCALAAAAAAAAAAAAAAAAA==",
                    "subType": "04"
                }
            },
            "path": "foo",
            "bsonType": "string"
        }
    ]
}