### Apache Airflow version

2.1.3 (latest released)

### Operating System

Debian Buster

### Versions of Apache Airflow Providers

_No response_

### Deployment

Managed Services (Astronomer, Composer, MWAA etc.)

### Deployment details

Running `quay.io/astronomer/ap-airflow:2.1.3-buster` Docker image

### What happened

In the task attributes view of a BashOperator, the `template_ext` row doesn't show any value, while I would expect to see `('.sh', '.bash')`.

![image](https://user-images.githubusercontent.com/6249654/131467817-8620f68d-0f3d-491d-ad38-cbf5b91cdcb3.png)


### What you expected to happen

Expect to see correct `template_ext` value

### How to reproduce

Run a BashOperator and browse to Instance Details --> scroll down to `template_ext` row

### Anything else

Spent a little time trying to figure out what could be the reason, but haven't found the problem yet. Therefore created issue to track.

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

airflow/serialization/serialized_objects.py
"""Helper function of depth first search for serialization.
        The serialization protocol is:
        (1) keeping JSON supported types: primitives, dict, list;
        (2) encoding other types as ``{TYPE: 'foo', VAR: 'bar'}``, the deserialization
            step decode VAR according to TYPE;
        (3) Operator has a special field CLASS to record the original class
            name for displaying in UI.
        """
        if cls._is_primitive(var):
            
            if isinstance(var, enum.Enum):
                return var.value
            return var
        elif isinstance(var, dict):
            return cls._encode({str(k): cls._serialize(v) for k, v in var.items()}, type_=DAT.DICT)
        elif isinstance(var, list):
            return [cls._serialize(v) for v in var]
        elif HAS_KUBERNETES and isinstance(var, k8s.V1Pod):
            json_pod = PodGenerator.serialize_pod(var)
            return cls._encode(json_pod, type_=DAT.POD)
        elif isinstance(var, DAG):
            return SerializedDAG.serialize_dag(var)
        elif isinstance(var, BaseOperator):
            return SerializedBaseOperator.serialize_operator(var)
        elif isinstance(var, cls._datetime_types):
            return cls._encode(var.timestamp(), type_=DAT.DATETIME)
        elif isinstance(var, datetime.timedelta):
            return cls._encode(var.total_seconds(), type_=DAT.TIMEDELTA)
        elif isinstance(var, Timezone):