public void validateFileExtensions() {
        validFileExtensions.forEach((subDir, __) -> validateInDir(subDir.toFile().toPath()));
    }
Builder for {@link com.yahoo.config.model.application.provider.FilesApplicationPackage}. Use
this to create instances in a flexible manner.
config-application-package/src/main/java/com/yahoo/config/model/application/provider/FilesApplicationPackage.java