I know about similar questions, I did almost everything they suggested in these questions, but my icon app haven't appeared so far.
Why?
I use latest iOS 8.3 on both device and simulator. While it works on simulator it doesn't work on device. However if I try to EDIT my Today extensions, the icon, indeed, IS VISIBLE on the list. It means that the icon asset is visible for Today.
Below are my screens from Device:
and Simulator:
This is my IcoApp set
My icon on device is working:
This is my icon.plist for extension:
For me it looks like a very big bug from Apple