I was able to set app icons by placing icon-40.png, icon-60.png,
icon-60@2x.png, icon-76.png and
icon-76@2x.png in the Special folder inside the Files folder and I correctly see the app icon on my test device. I can also see the correct icon in the Settings menu.
I don't see the correct icon, anyway, when performing a Spotlight search (I'm testing on an iPhone 6s): I see the standard B4i icon with the blue gradient.
I tried placing a icon-small-40.png and a
icon-small-40@2x.png in the same folder, but with no success.