byte[] download(File f) {
   if (!f.exists()) {
     IJ.error("Plugin Installer", "File not found: " + f);
     return null;
   }
   byte[] data = null;
   try {
     int len = (int) f.length();
     InputStream in = new BufferedInputStream(new FileInputStream(f));
     DataInputStream dis = new DataInputStream(in);
     data = new byte[len];
     dis.readFully(data);
     dis.close();
   } catch (Exception e) {
     IJ.error("Plugin Installer", "" + e);
     data = null;
   }
   return data;
 }