I am interested, what package/software/tool places previews into .local/share/previews? I found just one preview in there, and am interested about where it came from.
SnapOverflow
It's the Unity Dash. If you right-click on a file in the Dash, a bigger preview is created, which is placed in this folder to speed up loading the next time.
You can test this by right-clicking an image or video file in the Dash and then see the newly created thumbnail in the folder.