The SlackAPIFileOperator is not straight forward and it might be better to add an example DAG to demonstrate the usage.

tests/providers/slack/operators/test_slack.py
test_slack_conn_id = 'test_slack_conn_id'
        slack_api_post_operator = SlackAPIPostOperator(
            task_id='slack',
            username=self.test_username,
            slack_conn_id=test_slack_conn_id,
        )
        slack_api_post_operator.execute()
        expected_api_params = {
            'channel': "#general",
            'username': self.test_username,
            'text': 'No message has been set.\n'
            'Here is a cat video instead\n'
            'https://www.youtube.com/watch?v=J---aiyznGQ',
            'icon_url': "https://raw.githubusercontent.com/apache/"
            "airflow/main/airflow/www/static/pin_100.png",
            'attachments': '[]',
            'blocks': '[]',
        }
        assert expected_api_params == slack_api_post_operator.api_params
class TestSlackAPIFileOperator(unittest.TestCase):