'Add a content type for the part with *partname* and *content_type*,
using a default or override as appropriate.'

def _add_content_type(self, partname, content_type):

    ext = partname.ext
    if ((ext.lower(), content_type) in default_content_types):
        self._defaults[ext] = content_type
    else:
        self._overrides[partname] = content_type
