It's not an Aperture problem, or even a Mail one, but an Outlook one. If a plain text email is not showing up as plain text with attachments, then the issue is all on the recipients' side - bluntly, they don't know how to use their application.
Some folks have got around this by zipping their files. Others upload them to Dropbox and send a link to the other end. Most folks can manage a link...
I have "windows friendly attachments" checked, although that particular menu item is greyed out, and can't either be selected or de-selected.
That is funny, where do you access this flag?
It is in the "Edit" menu and at the bottom of the file chooser window.
Windows friendly does not mean, that the images will be prevented from being displayed inline, see this exerpt from take-control-of mail: http://www.ncmug.org/tips/mail_attachments.html
The only safe way to ensure that attachments will not be appear as inline attachments is to zip them and to send them as a compressed file.
I have experimented extensively with this issue. It is related to the recipitants email app, but in previous versions of Mail could be made successful by clicking on Format/Make Plain Text prior to sending. In Mail in Lion and ML, this change does not stick, and the message utlimately gets sent in Rich Text/HTML which is why the MS apps have problems.
The Make Plain Text strangely enough endures if you add a Signature following the attachments that is EMPTY but composed as text only plain text. This 'tricks" Mail and OSX to not change it back. I cooresponded with Apple engineering about this, and they say the use of Rich Text is necessary and helpful with Exchange Accounts. But this does not help the many who are not in Exchange Account Situations.
The use of Attachment Tamer in general can help this, but I don't use it, and it is not clear to me how it would work when invoking Mail sending from within Aperture (or iPhoto which suffers the same issue).
MS could probably solve the problem, but at last check even RTF produced in Outlook could be problematic to those using Outlook Express, or vice-versa.