srnd: fix multipart message parsing

contrib/backends/srndv2/src/srnd/store.go

@@ -632,7 +632,7 @@ func read_message_body(body io.Reader, hdr map[string][]string, store ArticleSto
 		body = io.TeeReader(body, wr)
 	}
 	boundary, ok := params["boundary"]
-	if content_type == "multipart/mixed" && ok {
+	if strings.HasPrefix(media_type, "multipart/") && ok {
 		partReader := multipart.NewReader(body, boundary)
 		for {
 			part, err := partReader.NextPart()