Problem with posting files to slack channel using Python

Question

So I need to send messages and post files to slack channels using Python.

I am able to send messages but I am getting error uploading files. The relevant piece of code looks likes this:

    response = self.client.chat_postMessage(
         channel='#notifications',
         text=message)

    filepath=f'./{file_name}'
    response = self.client.files_upload(
        channels='#notifications',
        file=filepath)

The message gets posted successfully, the file upload results in error:

slack.errors.SlackApiError: The request to the Slack API failed.
The server responded with: {'ok': False, 'error': 'not_in_channel'}

I have set the following permission in Slack OAuth and permissions screen:

incoming-webhook, chat:write, chat:write.public, files:write

I know that if I dont give chat:write.public I get same error while posting messages as well. However I am not able to find any files:write.public permission.

Has anyone encountered this error? Any insight is appreciated.