'Return content type for part identified by *partname*.'

def __getitem__(self, partname):

    if (not isinstance(partname, PackURI)):
        tmpl = "_ContentTypeMap    key    must    be    <type    'PackURI'>,    got    %s"
        raise KeyError((tmpl % type(partname)))
    if (partname in self._overrides):
        return self._overrides[partname]
    if (partname.ext in self._defaults):
        return self._defaults[partname.ext]
    tmpl = "no    content    type    for    partname    '%s'    in    [Content_Types].xml"
    raise KeyError((tmpl % partname))
