diff --git a/lib/security.js b/lib/security.js
index fba5a5c81..4fa8b6e4c 100644
--- a/lib/security.js
+++ b/lib/security.js
@@ -342,8 +342,8 @@ function messageFilter (message, client, isEdit) {
 
 function isAttachmentOk (a) {
   const isImage = a.width > 0 && a.height > 0
-  const url = a.url.toLowerCase()
-  const ext = path.extname(url)
+  const name = a.name
+  const ext = path.extname(name)
   return isImage || config.spam.allowedExtensions.includes(ext)
 }