Nautilus/Nemo draws an ugly frame border on thumbnails bigger than 128px #3
Description
In order to avoid the frame we need to cap the max size to 127px (or actually 52px to make the icon have the same size of native mime-icons).
This approach, however, has the drawback of affecting the image quality. I don't know if there's any other alternative, since the "framing" seems to be hardcoded:
Therefore, for now I will mark this issue as "wontfix".