code: public static File create(FileCreateParams params, RequestOptions options)
        throws StripeException {
      checkNullTypedParams(classUrl(File.class, Stripe.getUploadBase()), params);
      return create(params.toMap(), options);
    }
docs: To upload a file to Stripe, you’ll need to send a request of type {@code multipart/form-data}.
  The request should contain the file you would like to upload, as well as the parameters for
  creating a file.