The problem regards "folder view" plasmoid when enabling preview for its content: the resulting image is very aliased and with wrong aspect ratio. Seems that thumbnails are resized to match a square, regardless original image is landscape or portrait.

Here what I mean: http://i.imgur.com/8vqcSTp.png
on the left "folder view" while on the right dolphin showing the same content

Reproducible: Always

Steps to Reproduce:
1. Add a "folder view" to desktop
2. Select as source a folder containing, f.i., images or videos
3. Enable a preview engine

Actual Results:  
Thumbnail preview is scaled to match a square and the resulting image is aliased and distorted

Expected Results:  
A thumbnail with correct aspect ratio and smooth