code: public static File create(Map<String, Object> params, RequestOptions options)
        throws StripeException {
      return multipartRequest(RequestMethod.POST, classUrl(File.class, Stripe.getUploadBase()),
          params, File.class, 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.