Add 'keep_old_files' option.
I'm amazed to see that a 2.5 years old commit can merge cleanly into Paperclip. :)
Oh well, it's never too late to get something cool in ;)
Merge pull request #674 from teefax/keep_old_files
Add ':keep_old_files' option to `has_attached_file`
Set this option to `true` will make paperclip retain your file from the storage engine.