//var tmpath = path.join(os.tmpdir(), '1.txt');//模拟上传到临时目录的文件 
  //console.log(tmpath); 
  //var ext = ".txt";//上传后生成文件的后缀，一般和上传的文件后缀一致 
  //var ph = path.join('public/upload', Date.parse(new Date()).toString() + ext);//生成新的上传文件路径全称 
  //console.log(ph); 
  //var stream = fs.createWriteStream(ph);//创建一个可写流 
  //fs.createReadStream(tmpath).pipe(stream);//可读流通过管道写入可写流

fs.createWriteStream(path.join(os.tmpdir(), Math.random().toString()));

part.pipe(stream); 

const Koa = require('koa');
const app = new Koa();
const bodyParser = require('koa-bodyparser');
const koaBody = require('koa-body');
const router = require('./router');
// 输出请求路径，每次请求都会输出
app.use(async (ctx, next) => {
console.log(`Process ${ctx.request.method} ${ctx.request.url}...`);
await next();
});
// 文件上传，注意书写的位置很重要，否则无法上传
app.use(koaBody({ multipart: true }));
// 解析请求体
app.use(bodyParser());
// 使用路由
app.use(router());
app.listen(3000);
console.log('app started at port 3000...');

{ fields: { aaa: '123345' },    //Text类型的解析到fields中
 files:              //File类型的解析到files中
  { 
   abc: 
   File {
    domain: null,
    _events: {},
    _eventsCount: 0,
    _maxListeners: undefined,
    size: 12525,
    path: 'C:\\Users\\DANNYJ~1\\AppData\\Local\\Temp\\upload_d896dcc755fcd36156c6814aafb7685e',
    name: 'bamboo5.png',
    type: 'image/png',
    hash: null,
    lastModifiedDate: 2017-12-08T10:17:04.355Z,
    _writeStream: [Object] } 
   } 
}

  var file = ctx.request.body.files.abc  //传输文件的name是abc
  console.log(ctx.request.body)
  var tmpath= file['path'];
  var tmparr =file['name'].split('.');
  var ext ='.'+tmparr[tmparr.length-1];
  var newpath =path.join('./', parseInt(Math.random()*100) + Date.parse(new Date()).toString() + ext);
  console.log(tmpath);
  console.log(newpath);
  var stream = fs.createWriteStream(newpath);//创建一个可写流
  fs.createReadStream(tmpath).pipe(stream);//可读流通过管道写入可写流