Major Description
CssPackageResource#processResponse is never called if the extension isn't css. It works for .css because PackageResourceReference#getResource checks if the extension is css and then creates a CssPackageResource. For anything else than .css/.js a PackageResource is created.
Actually the overridden processResponse method in CssPackageResource (instead of PackageResource) should get called, but ResourceReferenceRegistry#createDefaultResourceReference installs a plain PackageResource which gets called later.