public static Map<String, FileNode> asFileNodeMap(Map<String, String> stringMap) {
        Map<String, FileNode> fileNodeMap = new LinkedHashMap<>();
        for (Map.Entry<String, String> e : stringMap.entrySet()) {
            fileNodeMap.put(e.getKey(), new FileNode(e.getValue()));
        }
        return Collections.unmodifiableMap(fileNodeMap);
    }
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