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