While WebP hasn't taken off in popularity as some had hoped for, it's still out there and slowly gaining traction in some circles. The BSD-licensed WebP image format derived from the VP8 video format is causing frustration among some in the FreeDesktop.org community due to its Internet media type.
Most implementations of WebP use "image/webp" as the MIME type for WebP image files, but the FreeDesktop.org developers responsible for the XDG shared-mime-info package that supplies MIME information, will not add the "image/webp" type. Those behind this FreeDesktop.org XDG component will not add the MIME type until it's been officially approved by the Internet Assigned Numbers Authority (IANA) even though most software is unofficially using image/webp.
There's FreeDesktop.org Bug 41155 that dates back to 2011 but in recent days has been quite active by those on both sides of the table about adding the WebP image type. As the latest activity, an image/x-webp MIME type has been added in Git, but not all are in favor of the "x-webp" MIME type.