file_download.php builds a local array of mime types that will be forced to display inline: $t_mime_force_inline

This array does not include 'image/webp', and cannot be overridden, so WebP images are forced to download. It would be better to allow them to display inline as per other image formats. Of the popular web browsers, only IE11 and Safari browsers cannot display WebP images, so I reckon it's no risk to allow WebP images to display inline always.

https://caniuse.com/#search=webp

If it is considered a risk, then perhaps $t_mime_force_inline can be made a configuration item that can be overridden in local configs for people who don't care about IE11 and Safari.