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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def post_file(self, url, filename, file_stream, *args, **kwargs):
 """Uploads file to provided url. Returns contents as text Args: **url**: address where to upload file **filename**: Name of the uploaded file **file_stream**: file like object to upload .. versionadded:: 0.3.2 **additional_headers**: (optional) Additional headers to be used with request Returns: string """

        res = self._conn.post(url, files={filename: file_stream},
                              headers=self._prepare_headers(**kwargs))
        if res.status_code == 200 or res.status_code == 201:
            return res.text
        else:
            return None