I don't know what or where you have found that documentation, but I think the EPT driver will always assume the print output should be attached - usually as an RTF or a PDF, although you can configure it for any print driver output. Leaving the PrtType blank will just default to PDF or RTF. Older version defaulted to RTF. I think newer ones default to PDF, but I could be mistaken on that.
The FSRSetFileAttachment is an internal API that is / was documented for consultants that might be developing a custom mail interface. The EPT driver uses that API to assign the attachment and can't be bypassed.
Page 92 has some information on configuring MPM email body output with no attachments.
You are right though, in other studio documentation it states PrtType will default to RTF.
FSRSetFileAttachment is briefly mentioned in this document as well.
Thank you all for the input thus far. I think the only issue I have left is the orientation of the email body. It appears centered...