Nuxt.js + Netlify form with input file

<template>
  <div class="question-form">
    <form name="ask-question"
          method="post"
          data-netlify="true"
          data-netlify-honeypot="bot-field"
          @submit.prevent="handleSubmit"
          enctype="multipart/form-data">

    <input type="hidden" name="form-name" value="ask-question" />

      <ul>
        <li>
          <label>
            Name:
            <input
              type="text"
              name="name"
              @input="ev => form.name = ev.target.value"
              >
          </label>
        </li>
        <li>
          <label>
            Upload:
            <input
              type="file"
              name="attach"
              ref="file"
              @change="addFile()"
              >
          </label>
        </li>
      </ul>
      <button type="submit" class="submit-button">Send</button>
    </form>
  </div>
</template>

<script>
import axios from "axios"

export default {
  data() {
    return {
      form: {
        name: "",
        attach: ""
      },
    };
  },
  methods: {
    addFile() {
      this.form.attach = this.$refs.file.files[0];
    },
    encode (data) {
        .map(
          key => `${encodeURIComponent(key)}=${encodeURIComponent(data[key])}`
        )
        .join("&"))
      return Object.keys(data)
        .map(
          key => `${encodeURIComponent(key)}=${encodeURIComponent(data[key])}`
        )
        .join("&");
    },
    handleSubmit() {
      const data = {
        "form-name": "ask-question",
        "name": this.form.name,
        "attach": this.form.attach
      }
      const axiosConfig = {
        header: { "Content-Type": "multipart/form-data" }
      };
      axios.post(
        "/",
        this.encode({
          data
        }),
        axiosConfig
      )
      .then(function (response) {
          console.log(response);
        })
      .catch(function (error) {
        console.log(error);
      });
    }
  }
};
</script>