Usage: no_upscale()
This filter tells thumbor not to upscale your images.
This means that if an original image is 300px width by 200px height and you ask for a 600x400 image, thumbor will still return a 300x200 image.
No arguments allowed.
http://thumbor-server/unsafe/300x200/filters:no_upscale()/some/image.jpg
/some/image.jpg){kind=link}