Running 9.0.2 (please don’t ask me why, haha). When I found out that thumbnails for RAW files are not available decided to implement them myself. Turned out to be very easy - just copied the function for generating video thumbnails and adjusted the code. It’s using exiftool as the tool to extract embedded preview file. Tested with my ARW files - works just fine. BUT! For the love of all programming gods, I can’t make it trigger the thumbnail generation automatically when viewing the folder through Web UI. In order to make the thumbnail appear I am forced to access a link like this:
https://mydomain.com/thumbnail/93d3aede-8aed-407e-8aac-ab58c1c282e3/48/myfolder/Photos/M3_07461.ARW
But that’s not useful, I want it to generate thumbnails by itself, like it does with images and videos (btw, it works just fine). I’ve looked through code multiple times and can’t find the place which is responsible for this. I’ve added arw to the list of image file extensions, but it didn’t help 
Any hint will be highly appreciated, I am soooo close to make it work!
Aaaand solved! Turned out editing source JS is useless unless you rebuild the assets, which I don’t want to do. So, I just edited directly the assets (a bit of a pain, due to the the fact that it’s one line inside). After that it finally works!
If anyone is interested in seeing the code - let me know!