### 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
elif len(dags) == 1:
            dag = dags.popitem()[1]
        else:
            raise AirflowException(
                "Tried to create relationships between tasks that don't have "
                "DAGs yet. Set the DAG for at least one "
                "task and try again: {}".format([self] + task_list)
            )
        if dag and not self.has_dag():
            
            
            self.dag = dag
            self.dag.task_group.add(self)
        for task in task_list:
            if dag and not task.has_dag():
                
                
                task.dag = dag
                task.dag.task_group.add(task)
            if upstream:
                task.add_only_new(task.get_direct_relative_ids(upstream=False), self.task_id, self.dag.dag_id)
                self.add_only_new(self._upstream_task_ids, task.task_id, task.dag.dag_id)
                if edge_modifier:
                    edge_modifier.add_edge_info(self.dag, task.task_id, self.task_id)
            else:
                self.add_only_new(self._downstream_task_ids, task.task_id, task.dag.dag_id)
                task.add_only_new(task.get_direct_relative_ids(upstream=True), self.task_id, self.dag.dag_id)
                if edge_modifier:
                    edge_modifier.add_edge_info(self.dag, self.task_id, task.task_id)
    def set_downstream(