If you want to see your attachment in the attachment tab you should use use
DocumentHelper.AddUnsortedAttachment(rootPage, "FileAttachment", filepath);
There are two types of attachments:
- Unsorted - added via the Properties -> Attachments tab; using this approach, you can easily add attachments to any document; all attachments added this way are taken as one group and displayed together using the web parts
- Grouped - added by defining a 'Document attachments' field for a document type and then uploading a file on a document's Form tab; for each Document attachments field, you can have an unlimited number of files attached; you can also define more fields of this type to have several groups of attachments which can be displayed separately on the live site