By default, Filerobot delivers the images as WebP. If the format is not supported by the client browser, it falls back to JPEG or PNG. If you wish to always keep the original format, you can use force_format=original. For more details, you can see Deliver images in the original format.
You can of course force a specific output format(s). More details can be found in the image formats section.
For troubleshooting purposes, you can add the ci_info=1 operation to view details about resizing and compression, example: /docs/hotel.jpg?force_format=png&ci_info=1