fix variable name confusion at AttachmentsController#thumbnail

This is suggested by Rails team.
https://github.com/rails/rails/issues/15471#issuecomment-45419147

git-svn-id: http://svn.redmine.org/redmine/trunk@13176 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/controllers/attachments_controller.rb

@@ -61,9 +61,9 @@ class AttachmentsController < ApplicationController
   end
 
   def thumbnail
-    if @attachment.thumbnailable? && thumbnail = @attachment.thumbnail(:size => params[:size])
-      if stale?(:etag => thumbnail)
-        send_file thumbnail,
+    if @attachment.thumbnailable? && tbnail = @attachment.thumbnail(:size => params[:size])
+      if stale?(:etag => tbnail)
+        send_file tbnail,
           :filename => filename_for_content_disposition(@attachment.filename),
           :type => detect_content_type(@attachment),
           :disposition => 'inline'