loadPreviews () {
    this.files.map(file => {
      if (!file.previewData && window && window.FileReader && /^image\//.test(file.file.type)) {
        const reader = new FileReader()
        reader.onload = e => Object.assign(file, { previewData: e.target.result })
        reader.readAsDataURL(file.file)
      }
    })
  }
load image previews for all uploads.