@Override
    public long lastModified() throws IOException {
        long lastModified = getFileForLastModifiedCheck().lastModified();
        if (lastModified == 0L) {
            throw new FileNotFoundException(getDescription() +
                                                    " cannot be resolved in the file system for resolving its last-modified timestamp");
        }
        return lastModified;
    }
This implementation checks the timestamp of the underlying File,
if available.

@see #getFileForLastModifiedCheck()
src/main/java/spark/resource/AbstractResource.java