def documentCover(self):
        """Cover page image

        :return: (file extension, file object) tuple.
        """
        rels_pth = os.path.join(self._cache_dir, "_rels", ".rels")
        rels_xml = lxml.etree.parse(xmlFile(rels_pth, 'rb'))
        thumb_ns = ns_map["thumbnails"]
        thumb_elm_xpr = "relationships:Relationship[@Type='%s']" % thumb_ns
        rels_xpath = lxml.etree.XPath(thumb_elm_xpr, namespaces=ns_map)
        try:
            cover_path = rels_xpath(rels_xml)[0].attrib["Target"]
        except IndexError:
            return None
        cover_fp = open(self._cache_dir + os.sep + cover_path, "rb")
        cover_type = imghdr.what(None, h=cover_fp.read(32))
        cover_fp.seek(0)
        # some MS docs say the type can be JPEG which is ok,
        # or WMF, which imghdr does not recognize...
        if not cover_type:
            cover_type = cover_path.split('.')[-1]
        else:
            cover_type = cover_type.replace("jpeg", "jpg")
        return (cover_type, cover_fp)
Cover page image

        :return: (file extension, file object) tuple.