public List<File> filterFiles(String ext) {
        List<File> f = new ArrayList<File>();
        for (String s : l) {
            File sf = new File(s);
            File parent = sf.getParentFile();
            if (!parent.exists()) {
                throw new IllegalArgumentException("Not found: " + parent);
            }
            for (File file : parent.listFiles(new WildcardFileFilter(sf.getName()))) {
                //TODO put config logger here.
                if (file.getName().endsWith(ext)) {
                    f.add(file);
                }
            }
        }
        return f;
    }