Download backup file from WebDAV (cloud only).

def backup_download(self, filename=None):
        """Download backup file from WebDAV (cloud only)."""
        if self.deploymentType != 'Cloud':
            logging.warning(
                'This functionality is not available in Server version')
            return None
        remote_file = self.backup_progress()['fileName']
        local_file = filename or remote_file
        url = self._options['server'] + '/webdav/backupmanager/' + remote_file
        try:
            logging.debug('Writing file to %s' % local_file)
            with open(local_file, 'wb') as file:
                try:
                    resp = self._session.get(url, headers=self._options['headers'], stream=True)
                except Exception:
                    raise JIRAError()
                if not resp.ok:
                    logging.error("Something went wrong with download: %s" % resp.text)
                    raise JIRAError(resp.text)
                for block in resp.iter_content(1024):
                    file.write(block)
        except JIRAError as je:
            logging.error('Unable to access remote backup file: %s' % je)
        except IOError as ioe:
            logging.error(ioe)
        return None