将图片文件转成BASE64格式

     html5Reader (file, item) {
        const reader = new FileReader()
        reader.onload = (e) => {
          this.$set(item, 'src', e.target.result)
        }
        reader.readAsDataURL(file)
      }

            const item = {
              name: list[i].name,
              size: list[i].size,
              file: list[i]
            }
            this.html5Reader(list[i], item)