react-hook-form file field TypeScript type

I'm building a contact form with react-hook-form and Typescript. Following official examples here, I've implemented all the fields but the multiple file field.

What type should I use for a multiple-file field?

import * as React from "react";
import { useForm } from "react-hook-form";

type FormData = {
  firstName: string;
  lastName: string;
  files: // <--- this is the type I'm looking for
};

export default function App() {
  const { register, setValue, handleSubmit, formState: { errors } } = useForm<FormData>();
  const onSubmit = handleSubmit(data => console.log(data));
  // firstName and lastName will have correct type

  return (
    <form onSubmit={onSubmit}>
      <label>First Name</label>
      <input {...register("firstName")} />
      <label>Last Name</label>
      <input {...register("lastName")} />
      <label>Files</label>
      <input type="file" multiple {...register("files")} />
      <button
        type="button"
        onClick={() => {
          setValue("lastName", "luo"); // ✅
          setValue("firstName", true); // ❌: true is not string
          errors.bill; // ❌: property bill does not exist
        }}
      >
        SetValue
      </button>
    </form>
  );
}