Def should be private!

But I am trying to do something like in carrierwave you override a method called extension_white_list so what I wanted to do was:

```ruby
def extension_to_content_type_white_list
  []
end

private

def contente_type_white_list
[ 'image/jpg',
  'image/jpeg',
  'image/gif',
  'image/png',
 ] + extension_to_content_type_white_list
end
```