thoughtbot/paperclip — pull request #1421 by ScotterC (2014-01-21 21:08:39+00:00)
Title: Adding Default Format for styles

Allows a lambda to be passed to decide what format a style should be processed as. This is really handy for when all your images are jpgs but you want a particular style to be an animated gif.

Cheers to @pelegr which is where this [code originated](https://github.com/pelegr/paperclip).