This is excellent news. I was always annoyed at the fact that Dolphin had to use its own thumbnailer API. Now apps can bundle one thumbnailer for their file formats that works with all file managers, which is great.
One thing that I always thought would be really interesting to see would be a Wine thumbnailer that would generate thumbnails using Windows thumbnails for Linux file managers. So for example installing Sketchup on Wine would give you skp thumbnails, since Sketchup is bundled with a Windows Explorer thumbnailer for those files. I know Wine already supports thumbnailer shell extensions in its built-in file manager, so it would just need to be able speak with Linux file managers.
Now I just hope to see something similar happen with service menus. Every file manager has its own unique format, but it's IMO the biggest obstacle to interoperability remaining with Linux file managers.