Express web app cannot download documents from NextCloud using OnlyOffice API

🍱 Features & apps onlyoffice
1

Hi, everyone, I’ve setup a NextCloud and OnlyOffice integration (both as Docker container) in my localhost by following this. I’m able to create a new document (named test.docx), open, and edit it within NextCloud. The autosave within NextCloud also worked as expected.
Now I’d like to be able to edit the same document (test.docx) from my Express app. I keep getting this error when I’m trying to load it:

image
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I can’t seem to find the correct document.url and editorConfig.callbackUrl for the initial setup. I copy the code from here.

These are things that I have tried:

  1. I can just go to http://localhost:3000/test.docx from Google Chrome and the test.docx will be downloaded correctly.
  2. I have tried to make a POST request from Postman to http://localhost:3000/track as in editorConfig.callbackUrl and it returns {"error":0} as expected.

Note: http://localhost:3000 is my local Express server (not in Docker), http://localhost:9080 is my OnlyOffice Document server address (in Docker), and I also have http://localhost:8080 Nginx server (in Docker) where I can access the NextCloud to open, edit, and save the .docx with the integrated OnlyOffice within the NextCloud

This is my index.html that is given from accessing http://localhost:3000/ from the browser:

<!DOCTYPE html>
<html style="height: 100%;">
<head>
    <title>ONLYOFFICE Api Documentation</title>
</head>
<body style="height: 100%; margin: 0;">
    <div id="placeholder" style="height: 100%"></div>
    <script type="text/javascript" src="http://localhost:9080/web-apps/apps/api/documents/api.js"></script>

    <script type="text/javascript">

        window.docEditor = new DocsAPI.DocEditor("placeholder",
            {
                "document": {
                    "fileType": "docx",
                    "key": "E7FAFC9C22A8",
                    "title": "Test.docx",
                    "url": "http://localhost:3000/test.docx"
                },
                "documentType": "text",
                "editorConfig": {
                    "callbackUrl": "http://localhost:3000/track",
                },
                "height": "100%",
                "width": "100%"
            });

    </script>
</body>
</html>

This is my server.js file in NodeJS server:

const app = express();
app.use(express.static(path.join(__dirname, 'files')));
app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.disable('x-powered-by');

app.get('/', (req, res) => {
	res.sendFile('index.html');
});

app.post('/track', (req, res) => {
	res.send({ error: 0 }); // for testing purpose
});

Does anyone know where I can get the correct document.url and editorConfig.callbackUrl? Or I make a mistake in other parts? If you need additional information just ask me.