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.
Sorry for the late reply.. I didn't notice This doesn't really work for me, I tried .iff or .png but this is what I get:
Maybe I'm misunderstanding..!seb