Fix Window Buttons (and xfwm4 Alt+Tab cycling) show thumnails with image files?

[Laurentius]

Although i have unticked "Show windows preview instead of icons when cycling" at Window manager tweaks > Compositor, when i open a pdf file (with xreader or evince) or an image file with xviewer, ristretto, or Gimp, the window buttons plugin and the Alt+Tab cycler shows a (small) thumbnail of the file, as if it was an application icon.
(Other kind of apps show the application icon, as expected. qpdfview shows it's icon, as expected.)

This does not happen at MX-17 with Evince or it's default image viewer (they show the icon)

I'm using Linuxmint 18.3 XFCE now, but i have noticed this behaviour from previous releases, at least 18.1 (i don't remember how 17.x behaved at this subject)

Why does this happen and how could it be fixed so that the compositor setting is honoured by this apps and show application-icon instead of file thumbnails?

[Laurentius]

Let me ask again. (i edited the tile and little of the body)