Slightly clean up patch detection code

Makes the code for detecting if an attachment is a patch or have a file
extension indicating that it's a patch slightly nicer.

Author: AndersAstrand

app/models/attachment.rb

@@ -6,11 +6,11 @@ class Attachment < ApplicationRecord
   after_destroy :update_topic_has_attachments
 
   def patch?
-    file_name&.ends_with?(".patch") || file_name&.ends_with?(".diff") || patch_content?
+    patch_extension? || patch_content?
   end
 
   def patch_extension?
-    file_name&.ends_with?(".patch") || file_name&.ends_with?(".diff")
+    file_name&.ends_with?('.patch', '.diff')
   end
 
   def decoded_body