Ausexpat;
sounds like you are talking about contextual-clicking on a photo attachment and you see limited options, including Export to iPhoto, but years after its introduction, Aperture is inexplicably absent from that contextual menu!
One solution I've just set up for myself for the same issue is to create a process for handling the attachments:
1) set up a new Project in Aperture called "imported from Mail"
2) Open Automator, and from the intro screen, click Application, then the Choose button
3) Click Photos in the Library in the far left pane
4) Drag Import Photos from the middle pane to the pane on the right
5) from the Choose Project pop-up menu, select the Project you created in step one
6) Save this Automator Application somewhere you will be able to find it
7) in Mail, select the photo(s) you want in Aperture, select Open With > Other... and choose the Application you created in step 6, then enjoy as you glide over to Aperture and see the photo in the Project!
Once you've set up the Application, it's a quick contextual (or "right-") click, Open Withe Other... click and you're done.
Good luck!