app.js

const koa = require("koa")
const koaBody = require('koa-body');

const app = new koa()
const router = new Router();
app.use(koaBody({
  multipart: true
}));

const uploadImg = require("./routes/api/upload-img")
router.use("/api", uploadImg);

app.use(cors());
app.use(router.routes()).use(router.allowedMethods())
const port = process.env.PORT || 5000
app.listen(port, () => {
  console.log(`server started osn ${port}`)
})

upload-img.js

const fs = require('fs')
const path = require('path')
const Router = require("koa-router");
const router = new Router();

router.post('/uploadfile', async (ctx, next) => {
  // 上***个文件
  console.log(ctx.request.files.file, '==========================')
  const file = ctx.request.files.file // 获取上传文件
  // 创建可读流
  const reader = fs.createReadStream(file.path);
  let filePath = path.join(__dirname, '../../public/upload/') + `/${file.name}`;
  // 创建可写流
  const upStream = fs.createWriteStream(filePath);
  // 可读流通过管道写入可写流
  reader.pipe(upStream);
  return ctx.body = "上传成功！";
});

module.exports = router.routes()

走postman，成功

upload下也有上传成功的图片了