How should I test this?

file_response.java

 
 public class FileRouter implements iResponse {
// omitted code 
 @Override
    public byte[] buildResponseBody(Map<String, String> request) throws IOException {
        String fileOneString = request.get("DIRECTORY") + request.get("URI");
        Path file = Paths.get(fileOneString);
        byte[] body      = Files.readAllBytes(filePath);
        return body;
    }