Where does NAS store thumbnails used by media server?

Any one know where Synology stores the thumbnails used by the media server?

this may be a red heiring… for he past several month I have been trying to figure out my my ds 220+ is not able show any of the files / content through the media server / DLNA. I had it working previously on my older nas.

After going down a few rabbit holes googling… starting to wounder if the thumbnails are failing to generate or have gotten corrupted. It would be nice to do a quick verification that they are there…

Of course photos works just fine so could be just a red heiring…