CURLcode curl_mime_subparts(curl_mimepart * part, curl_mime * subparts);
part is a handle to the multipart part.
subparts is a mime structure handle holding the subparts. After curl_mime_subparts succeeds, the mime structure handle belongs to the multipart part and must not be freed explicitly. It may however be updated by subsequent calls to mime API functions.
Setting a part's contents twice is valid: only the value set by the last call is retained. It is possible to unassign previous part's contents by setting subparts to NULL.