We recently had someone else show us a similar issue on a multi-site using WP Offload Media. The resulting testing showed that it was a mime-type not being attached. It’s probably a server/PHP configuration problem on your server. WP Offload Media looks to use the built-in WordPress mime-type detection routine which supports WebP, but not if your server isn’t configured to correctly use it. From our end, all we provide to WP Offload Media is a file path, there isn’t an option to provide any mime or content type so it’s not anything we can affect on our end.
That having been said, if you find something else going on, I would recommend contacting us directly and attaching your System Info from the support tab of the plugin settings: https://ewww.io/contact-us/