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
def setUp(self):
        self.test_username = 'test_username'
        self.test_channel = '#test_slack_channel'
        self.test_initial_comment = 'test text file test_filename.txt'
        self.test_filename = 'test_filename.txt'
        self.test_filetype = 'text'
        self.test_content = 'This is a test text file!'
        self.test_api_params = {'key': 'value'}
        self.expected_method = 'files.upload'
        self.expected_api_params = {
            'channel': self.test_channel,
            'initial_comment': self.test_initial_comment,
            'filename': self.test_filename,
            'filetype': self.test_filetype,
            'content': self.test_content,
        }
    def __construct_operator(self, test_token, test_slack_conn_id, test_api_params=None):