I'm not completely sure how to go about what you described, but one thing you could consider is using the deeplink module. With this module you can provide a link to the document you want to attach in the outlook email and then a user can download the file if they wish.
Here is a link to the deeplink module
Within the standard email with templates module from the appstore you can when you create the mail attach any Mendix filedocument as attachment to the mail. Or am I missing the point here.