Fix HTML part detection for some specific message structures (#1488992)

CHANGELOG

@@ -1,6 +1,7 @@
 CHANGELOG Roundcube Webmail
 ===========================
 
+- Fix HTML part detection for some specific message structures (#1488992)
 - Don't show fake address - phishing prevention (#1488981)
 - Fix forward as attachment bug with editormode != 1 (#1488991)
 - Fix LIMIT/OFFSET queries handling on MS SQL Server (#1488984)

program/lib/Roundcube/rcube_message.php

@@ -210,18 +210,20 @@ class rcube_message
                 if (!$recursive) {
                     $level = explode('.', $part->mime_id);
 
-                    // Skip if level too deep or part has a file name
+                    // Skip if part is an attachment
-                    if (count($level) > 2 || $part->filename) {
+                    if ($this->is_attachment($part)) {
                         continue;
                     }
 
-                    // HTML part can be on the lower level, if not...
+                    // Check if the part belongs to higher-level's alternative/related
-                    if (count($level) > 1) {
+                    while (array_pop($level) !== null) {
-                        array_pop($level);
+                        if (!count($level)) {
+                            return true;
+                        }
+
                         $parent = $this->mime_parts[join('.', $level)];
-                        // ... parent isn't multipart/alternative or related
                         if ($parent->mimetype != 'multipart/alternative' && $parent->mimetype != 'multipart/related') {
-                            continue;
+                            continue 2;
                         }
                     }
                 }