If you get this error: Invalid JSON response that means the server (server/upload.php) 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).
Navigate to the Console tab to see potential JavaScript errors and the Network tab to see the HTTP requests (make sure Record Network Log is on).
If you get server errors about the file size, make sure you change the settings for upload_max_filesize and post_max_size in php.ini.
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');