mime_delete collapse_alternatives doesn't find deeply nested multipart/alternative parts
Given a message with a structure like
multipart/signed
multipart/mixed
multipart/alternative
text/plain
text/html
(something else or not)
application/pgp-signature
mime_delete.collapse_alternatives only looks at the content types of the top level get_payload() list which is only the multipart/mixed and application/pgp-signature parts.