files.upload's file param doesn't accept bytes data

[seratch]

Description

Originally this is reported by @tplants at #728 (comment)

#730 resolved an issue with bytearray param but the change didn't support bytes properly.

# this is _bytes_ not a _bytearray_
file = io.BytesIO(bytes_like_object).getValue()

# This worked previously but does not work now.
res = client.files_upload(file=file, filename="test.xlsx", filetype="xlsx")

# This works now.
res = client.files_upload(file=io.BytesIO(file), filename="test.xlsx", filetype="xlsx")

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

[gburek-fastly]

After #771 was merged, the files_upload function signature still does not accept a bytes type object: https://github.com/slackapi/python-slack-sdk/blob/main/slack_sdk/web/client.py#L1458-L1460

    def files_upload(
        self, *, file: Union[str, IOBase] = None, content: str = None, **kwargs
    ) -> SlackResponse:

So the OP example produces a mypy error: Argument "file" to "files_upload" of "WebClient" has incompatible type "bytes"; expected "Union[str, IOBase, None]"

[seratch]

@gburek-fastly Thanks - I just created a new issue #901 for this.