Skip potentially animated GIF images when replacing them with WebP

Author: dtdesign

wcfsetup/install/files/lib/system/file/command/ReplaceWithWebpVariant.class.php

@@ -25,6 +25,12 @@ public function __invoke(): File
             return $this->file;
         }
 
+        // We cannot reliably detect if a GIF is animated without ImageMagick
+        // and the generated WebP variant will always be static.
+        if ($this->file->mimeType === 'image/gif') {
+            return $this->file;
+        }
+
         $command = new ReplaceFileSource(
             $this->file,
             $pathnameWebp,