Usage: format(image-format)
This filter specifies the output format of the image. The output must be one of: “webp”, “jpeg”, “gif” or “png”.
image-format
http://localhost:8888/unsafe/filters:format(webp)/https%3A%2F%2Fgithub.com%2Fthumbor%2Fthumbor%2Fraw%2Fmaster%2Fexample.jpg