How to reproduce:
1. Open Environman
2. Project should be open if its not, create a new one
3. In Hierarchy window, create a cube
4. Create a prefab from the cube
5. In Project window, right-click on the cube prefab -> Export Package...
6. Save the package
7. Observe the package icon in the saved path location

Expected result: the exported package file has Unity Editor icon and can be double clicked to import it to Unity
Actual result: the file has no icon

Tested on Hub 2.2.2

Reproducible with: 2017.4.36f1, 2018.4.15f1, 2019.2.6f1, 2019.3.0b2, 2020.1.0a18