'Return True iff the format exists on disk'

@read_api
def has_format(self, book_id, fmt):

    fmt = (fmt or u'').upper()
    try:
        name = self.fields[u'formats'].format_fname(book_id, fmt)
        path = self._field_for(u'path', book_id).replace(u'/', os.sep)
    except:
        return False
    return self.backend.has_format(book_id, fmt, name, path)
