Config fields of "file" type don't show overridden icon

[erikhansen]

Issue

File config fields don't show icon, even when they are overridden:

Steps to reproduce

1. Go to STORES > Configuration > GENERAL > Design > Emails
2. Change Store View to a scope other than "Default Config"
3. Upload an image
4. Change back to "Default Config" scope. You will see there is no icon present.