I don't know how Servlet's and the like work however i can give you a rundown of what you need to do.
In DemoServlet you need to take in the input of the Image_Name field and make that one of your parameters of FileUpload
public static boolean processFile(String path, FileItemStream item, String fileName){
//Method Code
}
Because currently your processFile method is taking the name of the file from your FileItemStream. You need to change it from that to your actual fileName
File savedFile = new File(f.getAbsolutePath() + File.separator + item.getName());
to
File savedFile = new File(f.getAbsolutePath() + File.separator + fileName + ".png");