ダサい。助けてくれ……できれば async/await で……→こんな感じか？

base64.js

export function encode (blob) {
  return new Promise((resolve) => {
    const r = new FileReader()
    r.onload = () => {
      const dataURL = r.result
      const base64 = dataURL.split(',', 2)[1]
      resolve(base64)
    }
    r.readAsDataURL(blob)
  })
}

export default {
  encode
}

base64use.js

import Base64 from './base64'

function createProduct (values) {
  return async (dispatch, getState) => {
    const tempfiles = getState().productsReducer.tempfiles
    let imageObjects = []
    for (const f of tempfiles) {
      imageObjects.push({
        contentType: f.type,
        base64Str: await Base64.encode(f)
      })
    }
    const body = {
      ...values,
      price: Number(values.price),
      imageObjects: imageObjects
    }

    return API.post('/api/products', body)
      .then(json => dispatch({
        type: 'CREATE_PRODUCT',
        payload: json
      })).catch(err => dispatch({
        type: 'CREATE_PRODUCT_ERROR',
        payload: err,
        error: true
      }))
  }
}