Last modified: 2013-01-25 11:57:37 UTC
Comparing the thumbnail of a rgb image: https://upload.wikimedia.org/wikipedia/commons/thumb/archive/7/7c/20121219183748!Van_Vechten_-_Willa_Cather.jpg/93px-Van_Vechten_-_Willa_Cather.jpg with the one of a greyscale jpeg: https://upload.wikimedia.org/wikipedia/commons/thumb/archive/7/7c/20121219183728!Van_Vechten_-_Willa_Cather.jpg/93px-Van_Vechten_-_Willa_Cather.jpg reveals that greyscale jpeg thumbnails do not get sharpened.
[ Adding to tracking bug 41371 ]
Confirming.