NodeJS请求主体返回空{}

I have this javascript function that is supposed to upload a .json file to the server.

function project_save_confirmed(input) {
    if ( input.project_name.value !== _onco_settings.project.name ) {
        project_set_name(input.project_name.value);
    }

    // onco project
    var _onco_project = { '_onco_settings': _onco_settings,
        '_onco_img_metadata': _onco_img_metadata,
        '_onco_attributes': _onco_attributes };

    var filename = input.project_name.value + '.json';
    var json = JSON.stringify(_onco_project);
    var data_blob = new Blob( [JSON.stringify(_onco_project)],
        {type: 'text/json;charset=utf-8'});

    //save_data_to_local_file(data_blob, filename);
    upload_json_to_server(json, filename);

    user_input_default_cancel_handler();
}

async function upload_json_to_server(data_blob, filename) {
    console.log(data_blob);
    $.ajax({
        type: 'POST',
        url: 'http://localhost:3000/upload',
        body: data_blob,
        contentType: 'text/json'
    }).done(function (data) {
        console.log(data);
    });
}

On my server I have the route corresponding to the file upload

const express = require('express');
const fileUpload = require('express-fileupload');
const app = express();
const fs = require('fs');
const bodyParser = require("body-parser");

app.use(function(req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
    next();
});

app.use(bodyParser.json({
    extended: true
}));

app.post('/upload', function(req, res) {
    console.log(req.body);
    fs.writeFileSync('./data/oncofinder-' + Date.now() +'.json', req); //default: 'utf8'

});

app.listen(3000,function(){
    console.log("Working on port 3000");
});

But on my console.log(req.body) the only thing I get is {}.

EDIT:

I'm passing the JSON as plain text. On the at console.log(data_blob) it prints exactly what I want to save. I don't understand how the body can be empty since I'm printing right before I send it.

Any idea what I'm doing wrong?

If you want to upload files to backend, you should try multer. It's really good and if you need I can post examples