As of 2016 maya has the option to include a thumbnail via the export/save file dialog. This is then used in the new asset browser.
I was wondering if I can save a custom file that I generate myself in a tool. I didn’t find any flags for the file command so I wonder how it works…
Try creating an additional file in the same folder “your-scene-name.mb.swatch”, where it’s an .IFF file, just with a “.swatch” extension. Other image formats might work as well.