readFile.ts

const uploadFile = ({
  event,
  onComplete
}: {
  event: ChangeEvent<HTMLInputElement>;
  onComplete: (file: File, base64File: string) => void;
}) => {
  event.preventDefault();

  let base64File: string;
  const file: File | null = event.target.files ? event.target.files[0] : null;

  if (file) {
    const reader = new FileReader();

    reader.readAsDataURL(file);

    reader.onload = () => {
      base64File = btoa(reader.result as string);
      onComplete(file, base64File);
    };
  }
};