### 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
"""Getting the content of files for template_field / template_ext"""
        if self.template_ext:
            for field in self.template_fields:
                content = getattr(self, field, None)
                if content is None:
                    continue
                elif isinstance(content, str) and any(content.endswith(ext) for ext in self.template_ext):
                    env = self.get_template_env()
                    try:
                        setattr(self, field, env.loader.get_source(env, content)[0])
                    except Exception as e:
                        self.log.exception(e)
                elif isinstance(content, list):
                    env = self.dag.get_template_env()
                    for i in range(len(content)):
                        if isinstance(content[i], str) and any(
                            content[i].endswith(ext) for ext in self.template_ext
                        ):
                            try:
                                content[i] = env.loader.get_source(env, content[i])[0]
                            except Exception as e:
                                self.log.exception(e)
        self.prepare_template()
    @property
    def upstream_list(self) -> List["BaseOperator"]: