Ok, well, if that's the case..
Click on "Go Advanced" just under the reply box at the bottom of this page. On the page you go to, down a little ways you'll see a box that says "Manage Attachments". Click on that, choose "Browse" in the box that pops up, select the map you want to attach, and then hit "Upload".
Now, when you make your reply, the map file will be attached to your message for everyone to download.