I assume you are using the fileupload control on the smart form and that you have a smart object that was generated from the table where you want to store this information?
We will store the attached file in SQL as a base64 string, our file attachment control does this conversion when create/update/read methods are executed so you shouldnt have to worry about doing this manually in smart forms.
When you create the table where you want to store the attachmetns I would recommend using the type ntext/memo so that we have a field big enogh to store the poteintially long base 64 string representation of the file attachment.