WebP is a new image format being promoted by Google.
By default WordPress (and therefor Postie) doesn’t support WebP images attached to emails. However, it is quite easy to add support.
First add WebP support to WordPress by installing the wp-webp plugin.
The second step is to ensure your web server is sending the correct MIME type for *.webp images (image/webp). A little Googling will explain how to check/add MIME types for your server. If you are using shared hosting contact your hoster to see if it is set up correctly.