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://localhost:8888/unsafe/filters:no_upscale()/https%3A%2F%2Fgithub.com%2Fthumbor%2Fthumbor%2Fraw%2Fmaster%2Fexample.jpg