With Xcode 8 there is no way to select an app icon source under General for app extensions but with Xcode 8.1 I am not seeing "Asset Catalog App Icon Set Name" under build settings any more even if I set ASSETCATALOG_COMPILER_APPICON_NAME it still doesn't work what needs to be set in Xcode 8.1 for an app extension icon?
Xcode 8.1 App Extension Icon
AFAIK, extension always uses main app icon. You can't set another one.