Wrap FileReader to Promise

file-to-base64.ts

export function convertFileToBase64(file: File): Promise<string> {
  return new Promise((resolve, reject) => {
    const reader: FileReader = new FileReader();
    reader.onload = () => {
      if (typeof reader.result === 'string') {
        resolve(reader.result);
      } else {
        reject(new Error(`Reading file result should be base64 string. Result: ${reader.result}`));
      }
    };
    reader.onerror = error => reject(error);
    reader.readAsDataURL(file);
  });
}