Custom icon not shown in Thumbnail view

[hlhp]

Custom icon only shown in detail and list view.
In thumbnail view, it only show the default one.

[Mathias (Author)]

Not sure what you mean with custom icon. 

[hlhp]

Not sure what you mean with custom icon.

I meant the icons in...
Configuration> File Type Setup > Icons

The new icons I use have all the necessary resolutions(16x16 24x24 32x32 48x48 256x256)
But still not shown in thumbnail view.