關(guān)于fetch api的文件上傳問題,為何請(qǐng)求體里什么內(nèi)容都沒有

這是代碼，明明已經(jīng)把file賦給body，可是在瀏覽器發(fā)送請(qǐng)求時(shí)根本沒有requestpayload，后端也看不見數(shù)據(jù)，請(qǐng)問這是為何？

import React, { Component } from 'react';


let a=1;
class Upload extends Component{

    state = {
        file:null
    }

    handleFileClick(e) {
        this.setState({file: e.target.files[0]});
      }

      handleSubmitClick(e) {
          console.log(this.state.file);
        fetch("http://localhost:8080//upload.do",{
                    method: 'POST',
                    headers:{
                        'Access-Control-Allow-Origin':'*',
                        'Content-Type': 'multipart/form-data;boundary=56423498738365'
                    },
                    mode: 'cors',
                    body: this.state.file
                });
      }

    render(){
        return (
            <div>
                <input type="file" onChange={this.handleFileClick.bind(this)}/>
                <input type="button" value="123" onClick={this.handleSubmitClick.bind(this)}/>
            </div>
        );
    }
}
export default Upload;