'Mapping of manifest media-type to list of canonical names of that media-type'

@property
def manifest_type_map(self):

    ans = defaultdict(list)
    for item in self.opf_xpath(u'//opf:manifest/opf:item[@href    and    @media-type]'):
        ans[item.get(u'media-type').lower()].append(self.href_to_name(item.get(u'href'), self.opf_name))
    return {mt: tuple(v) for (mt, v) in ans.iteritems()}
