Merge branch '6.1' of https://github.com/WoltLab/WCF into 6.1

Author: BurntimeX

wcfsetup/install/files/lib/system/html/metacode/upcast/AttachMetacodeUpcast.class.php

@@ -59,10 +59,10 @@ public function upcast(\DOMElement $element, array $attributes): void
 
     private function isThumbnailWidth(Attachment $attachment, string|bool|int $width): bool
     {
-        if ($width === 'auto' || $width === false) {
+        if ($width === 'auto' || $width === false || $width === 'false') {
             return false;
         }
-        if ($width === true || $width === 0) {
+        if ($width === true || $width === 'true' || $width === 0) {
             return true;
         }
         return !($width > $attachment->thumbnailWidth);