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**Apache Airflow version**: 2.0.1

**Environment**: Docker

- **Cloud provider or hardware configuration**: Local file system
- **OS** (e.g. from /etc/os-release): Arch Linux
- **Kernel** (e.g. `uname -a`): 5.11.5-arch1-1

**What happened**:

I tried to post a file from a long Python string to a Slack channel through the SlackAPIFileOperator.
I defined the operator this way:
```
SlackAPIFileOperator(
            task_id="{}-notifier".format(self.task_id),
            channel="#alerts-metrics",
            token=MY_TOKEN,
            initial_comment=":warning: alert",
            filename="{{ ds }}.csv",
            filetype="csv",
            content=df.to_csv()
        )
```

Task failed with the following error:

```
DEBUG - Sending a request - url: https://www.slack.com/api/files.upload, query_params: {}, body_params: {}, files: {}, json_body: {'channels': '#alerts-metrics', 'content': '<a long pandas.DataFrame.to_csv output>', 'filename': '{{ ds }}.csv', 'filetype': 'csv', 'initial_comment': ':warning: alert'}, headers: {'Content-Type': 'application/json;charset=utf-8', 'Authorization': '(redacted)', 'User-Agent': 'Python/3.6.12 slackclient/3.3.2 Linux/5.11.5-arch1-1'}
DEBUG - Received the following response - status: 200, headers: {'date': 'Thu, 18 Mar 2021 13:28:44 GMT', 'server': 'Apache', 'x-xss-protection': '0', 'pragma': 'no-cache', 'cache-control': 'private, no-cache, no-store, must-revalidate', 'access-control-allow-origin': '*', 'strict-transport-security': 'max-age=31536000; includeSubDomains; preload', 'x-slack-req-id': '0ff5fd17ca7e2e8397559b6347b34820', 'x-content-type-options': 'nosniff', 'referrer-policy': 'no-referrer', 'access-control-expose-headers': 'x-slack-req-id, retry-after', 'x-slack-backend': 'r', 'x-oauth-scopes': 'incoming-webhook,files:write,chat:write', 'x-accepted-oauth-scopes': 'files:write', 'expires': 'Mon, 26 Jul 1997 05:00:00 GMT', 'vary': 'Accept-Encoding', 'access-control-allow-headers': 'slack-route, x-slack-version-ts, x-b3-traceid, x-b3-spanid, x-b3-parentspanid, x-b3-sampled, x-b3-flags', 'content-type': 'application/json; charset=utf-8', 'x-envoy-upstream-service-time': '37', 'x-backend': 'files_normal files_bedrock_normal_with_overflow files_canary_with_overflow files_bedrock_canary_with_overflow files_control_with_overflow files_bedrock_control_with_overflow', 'x-server': 'slack-www-hhvm-files-iad-xg4a', 'x-via': 'envoy-www-iad-xvw3, haproxy-edge-lhr-u1ge', 'x-slack-shared-secret-outcome': 'shared-secret', 'via': 'envoy-www-iad-xvw3', 'connection': 'close', 'transfer-encoding': 'chunked'}, body: {'ok': False, 'error': 'no_file_data'}
[2021-03-18 13:28:43,601] {taskinstance.py:1455} ERROR - The request to the Slack API failed.
The server responded with: {'ok': False, 'error': 'no_file_data'}
```

**What you expected to happen**:

I expect the operator to succeed and see a new message in Slack with a snippet of a downloadable CSV file.

**How to reproduce it**:

Just declare a DAG this way:
```
from airflow import DAG
from airflow.providers.slack.operators.slack import SlackAPIFileOperator
from pendulum import datetime


with DAG(dag_id="SlackFile",
         default_args=dict(start_date=datetime(2021, 1, 1), owner='airflow', catchup=False)) as dag:

    SlackAPIFileOperator(
        task_id="Slack",
        token=YOUR_TOKEN,
        content="test-content"
    )
```

And try to run it.

**Anything else we need to know**:

This seems to be a known issue: https://apache-airflow.slack.com/archives/CCQ7EGB1P/p1616079965083200

I workaround it with this following re-implementation:

```
from typing import Optional, Any

from airflow import AirflowException
from airflow.providers.slack.hooks.slack import SlackHook
from airflow.providers.slack.operators.slack import SlackAPIOperator
from airflow.utils.decorators import apply_defaults


class SlackAPIFileOperator(SlackAPIOperator):
    """
    Send a file to a slack channel
    Examples:
    .. code-block:: python
        slack = SlackAPIFileOperator(
            task_id="slack_file_upload",
            dag=dag,
            slack_conn_id="slack",
            channel="#general",
            initial_comment="Hello World!",
            file="hello_world.csv",
            filename="hello_world.csv",
            filetype="csv",
            content="hello,world,csv,file",
        )
    :param channel: channel in which to sent file on slack name (templated)
    :type channel: str
    :param initial_comment: message to send to slack. (templated)
    :type initial_comment: str
    :param file: the file (templated)
    :type file: str
    :param filename: name of the file (templated)
    :type filename: str
    :param filetype: slack filetype. (templated)
        - see https://api.slack.com/types/file
    :type filetype: str
    :param content: file content. (templated)
    :type content: str
    """

    template_fields = ('channel', 'initial_comment', 'file', 'filename', 'filetype', 'content')
    ui_color = '#44BEDF'

    @apply_defaults
    def __init__(
            self,
            channel: str = '#general',
            initial_comment: str = 'No message has been set!',
            file: Optional[str] = None,
            filename: str = 'default_name.csv',
            filetype: str = 'csv',
            content: Optional[str] = None,
            **kwargs,
    ) -> None:
        if (content is None) and (file is None):
            raise AirflowException('At least one of "content" or "file" should be defined.')
        self.method = 'files.upload'
        self.channel = channel
        self.initial_comment = initial_comment
        self.file = file
        self.filename = filename
        self.filetype = filetype
        self.content = content
        super().__init__(method=self.method, **kwargs)

    def execute(self, **kwargs):
        slack = SlackHook(token=self.token, slack_conn_id=self.slack_conn_id)
        args = dict(
            channels=self.channel,
            filename=self.filename,
            filetype=self.filetype,
            initial_comment=self.initial_comment
        )
        if self.content is not None:
            args['content'] = self.content
        elif self.file is not None:
            args['file'] = self.content
        slack.call(self.method, data=args)

    def construct_api_call_params(self) -> Any:
        pass
```

Maybe it is not the best solution as it does not leverage work from `SlackAPIOperator`.
But at least, it fullfill my use case.

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):