Using Paint.Net 3.5.10 with your 1.0.0.3 Version.
No Preview available and when saving an unknown error message pops up.
Also cant open the example files from the latest WebP Codec for Windows (v0.17) package wich is
using the libwebp 0.2.0.
Try it with a 320x240 photo, bmp & jpg.
Win7 x64