[BUG] `overwrite=False` is not protecting local files, they are overwritten anyway

[karolzlot]

    def downloadAllAttachments(self, downloadFolder=".", overwrite=True):
        """Download all of the attachments in this message to the local folder ``downloadFolder``. If ``overwrite`` is
        ``True``, existing local files will be overwritten by attachments with the same filename."""

overwrite=False doesn't overwrite only if duplicate filenames are in attachements to mail. Contrary to doctring it doesn't check if local files exists already or not.