If you get an error with this message:
Invalid response that means the server is not sending a valid JSON fromat. This may be caused from a PHP error.
You can use the browser dev tools to see the actual errors and response from server.
In Google Chrome, right-click and select Inspect, or use the keyboard shortcut: Ctrl+Shift+I (or Cmd+Opt+I on Mac).
If you get errors in the Console tab about the Origin request make sure in
server/upload_*.php you accept the request from where you send it by adding:
header('Access-Control-Allow-Origin: yourwebsite.com'); header('Access-Control-Allow-Origin: www.yourwebsite.com');
If you get server errors about the file size, make sure you change the settings for
post_max_size in php.ini