Return an iterator over the non-main parts of a multipart.
Skip the first of each occurrence of text/plain, text/html,
multipart/related, or multipart/alternative in the multipart (unless
they have a 'Content-Disposition: attachment' header) and include all
remaining subparts in the returned iterator. When applied to a
multipart/related, return all parts except the root part. Return an
empty iterator when applied to a multipart/alternative or a
non-multipart.