Is there a special reason you are not using the Kentico Form feature?
The upload file is a out of box feature in the Kentico Form. You can add it in the Form builder or you can manually add it as a File field type and use a Upload file form control. Then in the E-mail notification (that's going to send the email with all the data to some email address you specify), check attach uploaded document box so the attachment file can be emailed as well.
Also, found this Adding attachment to email via Kentico Email .SendEmail method which may help you if you wants the API way.