FIX: add_attachments signature

lib/email_sender_extensions.rb

@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
 module DiscourseEncrypt::EmailSenderExtensions
-  def add_attachments(post)
-    return if post.is_encrypted?
+  def add_attachments(*posts)
+    return if posts.any?(&:is_encrypted?)
     super
   end
 end