Load models in batches, to make this more practical for large datasets.

Note: find_each doesn't support order because it already specifies it.
And it happens to be 'id' as well.

lib/paperclip.rb

@@ -138,7 +138,7 @@ module Paperclip
     def each_instance_with_attachment(klass, name)
       unscope_method = class_for(klass).respond_to?(:unscoped) ? :unscoped : :with_exclusive_scope
       class_for(klass).send(unscope_method) do
-        class_for(klass).find(:all, :order => 'id').each do |instance|
+        class_for(klass).find_each do |instance|
           yield(instance) if instance.send(:"#{name}?")
         end
       end