### 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
super().__init__(*args, **kwargs)
        
        
        self._task_type = 'BaseOperator'
        
        self.ui_color = BaseOperator.ui_color
        self.ui_fgcolor = BaseOperator.ui_fgcolor
        self.template_fields = BaseOperator.template_fields
        self.operator_extra_links = BaseOperator.operator_extra_links
    @property
    def task_type(self) -> str:
        
        
        return self._task_type
    @task_type.setter
    def task_type(self, task_type: str):
        self._task_type = task_type
    @classmethod
    def serialize_operator(cls, op: BaseOperator) -> Dict[str, Any]:
        """Serializes operator into a JSON object."""
        serialize_op = cls.serialize_to_json(op, cls._decorated_fields)
        serialize_op['_task_type'] = op.__class__.__name__
        serialize_op['_task_module'] = op.__class__.__module__
        
        serialize_op['_is_dummy'] = op.inherits_from_dummy_operator
        if op.operator_extra_links:
            serialize_op['_operator_extra_links'] = cls._serialize_operator_extra_links(