The post parameter body in nodejs indicates an error

Thank you. Here's the thing. I'm learning nodejs and I'm having a problem with image uploading.

<input type="file" name="file" id="file">
    <button id="btn">上传</button>
    <img src="" alt="" id="img">
    <script>
        let file = document.querySelector('#file')
        let btn = document.querySelector('#btn')
        let img = document.querySelector('#img')
        btn.addEventListener('click',function(){
            console.log(file.files[0]);
            let files_data = file.files[0]
            let fileReader = new FileReader()
            fileReader.readAsDataURL(files_data)
            let base64;
            let date = new Date()
            let time = date.toJSON()
            fileReader.onloadend = function(e){
                base64 = e.target.result
                console.log('base641',base64);
                img.src = e.target.result
                console.log(typeof e.target.result);
                let xml = new XMLHttpRequest()
                xml.open('POST','http://localhost:3000/api/swiper',true)
                xml.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
                xml.onload = function(){

                }
                console.log('base64',base64);
                let obj = {
                    swi_url:base64,
                    swi_qiyong:false,
                    swi_type:'活动',
                    swi_time:time
                }
                xml.send(JSON.stringify(obj))
            }
        })

nodejs code:

const express = require('express')
const router = express.Router()
const multer = require('multer')
const mysql = require('../model/index')
const path = require('path')
const dizhi = path.join(__dirname,'../upload')
router.post('/',async(req,res) => {
    const data = req.body
    console.log(data);
})
module.exports = router

I converted the image into base64 format data in the front-end code, and then passed it to the back end through post request. I also received it through req.body, but it could not point to one of the data, and it would give an error if I pointed to it, and it could not point to the body parameter data of json.parent. I would like to ask you where the code is wrong.
Just started learning nodejs, asking really good questions baici, thanks for understanding