I don't see this called anywhere with attachment = false, so maybe you could omit the argument, rename it to setAttachmentNode() and only take attachmentNode argument?

Then you can just check if mAttachmentNode is non-null to determine whether you have an attachment or not?

get rid of mAttachment: +1

but renaming is not a good idea,because it is not a attachmentNode of the messagepart - isAttachment is a property of the messagePart itself.
Often the attachmentNode is the parent or grandpartent of the messagePart itself (because, it is the first child kmime of the email).

So I thing renaming it to setAttachmentNode soulds like, it is a attachmentNode of the messagepart.