npm install multy
multy will add all parts to req.body or ctx.body.
Parts is added in EXACTLY same order as received, important to mix of file and text in same form.
  var Multy = require('multy')
  app.use(Multy(options))options is same as busboy
// on koa v2, works as well in express with req.body
const fs = require('fs')
const Multy = require('multy')
const app = new (require('koa'))()
const router = require('koa-Router')()
app.use(router)
router.use(Multy())
router.post('/', ctx => {
  const { file } = ctx.request.body
  const stream = fs.createWriteStream('potato.jpg')
  ctx.body = 'potato'
  return new Promise((resolve, reject) => {
    file
      .pipe(stream)
      .on('close', _ => resolve())
      .on('error', reject)
  })
})
app.listen(3000)