public static Map<String, OptionalPathNode> asOptionalPathNodeMap(Map<String, Optional<FileReference>> fileReferenceMap) {
        Map<String, OptionalPathNode> pathNodeMap = new LinkedHashMap<>();
        for (Map.Entry<String, Optional<FileReference>> e : fileReferenceMap.entrySet()) {
            pathNodeMap.put(e.getKey(), new OptionalPathNode(e.getValue()));
        }
        return Collections.unmodifiableMap(pathNodeMap);
    }
Special case for file type, since FileNode param type (FileReference) is not same as type (String) in config builder
config-lib/src/main/java/com/yahoo/config/LeafNodeMaps.java