If the document you are sending has an overlay, the overlay is displayed on the attachment only if the overlay is displayed by default when you open the document in OnBase.
If you are sending an XML document, the XML stylesheet is applied to the document and it will be converted to HTML.
If you are sending an E-Form, the E-Form is sent as HTML.
If you are sending a Lotus Notes document with a stylesheet, the stylesheet is applied to the document and it will be converted to HTML. Lotus Notes documents without configured stylesheets are sent in their native format.