def _main(self, client, bucket, key, extra_args):
        """
        :param client: The client to use when calling CreateMultipartUpload
        :param bucket: The name of the bucket to upload to
        :param key: The name of the key to upload to
        :param extra_args: A dictionary of any extra arguments that may be
            used in the intialization.

        :returns: The upload id of the multipart upload
        """
        # Create the multipart upload.
        response = client.create_multipart_upload(
            Bucket=bucket, Key=key, **extra_args)
        upload_id = response['UploadId']

        # Add a cleanup if the multipart upload fails at any point.
        self._transfer_coordinator.add_failure_cleanup(
            client.abort_multipart_upload, Bucket=bucket, Key=key,
            UploadId=upload_id
        )
        return upload_id
:param client: The client to use when calling CreateMultipartUpload
        :param bucket: The name of the bucket to upload to
        :param key: The name of the key to upload to
        :param extra_args: A dictionary of any extra arguments that may be
            used in the intialization.

        :returns: The upload id of the multipart upload