This would an amazing enhancement for attachments, especially for sites that use many large pictures.
GD (PHP 5.5) and ImageMagick are supporting WebP. This could be made as option available if PHP 5.5+ is detected. Many of us already use PHP 5.5 anyways.
WebP is a new image format that provides lossless and lossy compression for images on the web. WebP lossless images are 26% smaller in size compared to PNGs. WebP lossy images are 25-34% smaller in size compared to JPEG images at equivalent SSIM index. WebP supports lossless transparency (also known as alpha channel) with just 22% additional bytes. Transparency is also supported with lossy compression and typically provides 3x smaller file sizes compared to PNG when lossy compression is acceptable for the red/green/blue color channels. Webmasters and web developers can use the WebP image format to create smaller and richer images that can help make the web faster.