### 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/models/baseoperator.py
"""
        Get list of the direct relatives to the current task, upstream or
        downstream.
        """
        if upstream:
            return self.upstream_list
        else:
            return self.downstream_list
    def __repr__(self):
        return "<Task({self.task_type}): {self.task_id}>".format(self=self)
    @property
    def task_type(self) -> str:
        """@property: type of the task"""
        return self.__class__.__name__
    def add_only_new(self, item_set: Set[str], item: str, dag_id: str) -> None:
        """Adds only new items to item set"""
        if item in item_set:
            self.log.warning('Dependency %s, %s already registered for DAG: %s', self, item, dag_id)
        else:
            item_set.add(item)
    @property
    def roots(self) -> List["BaseOperator"]:
        """Required by TaskMixin"""
        return [self]
    @property
    def leaves(self) -> List["BaseOperator"]:
        """Required by TaskMixin"""
        return [self]
    def _set_relatives(