fabiosistemi Not currently possible. First of all, the SVG elements in Files Gallery are created dynamically, with text, and are scaleable for each different layout. We can't just load any external graphic into there instead and expect it to work like the SVG.
The only way to implement something like that, would be to assume the item is an image if there is a preview image (icon) available for it. Then it would behave like an image in the layout, scaling and cropping the loaded custom icon within, as if it was an image. I might implement something like that. We can't just replace the advanced generated SVG elements.