I added mine to the EDIT menu.
Open a new message (or reply to one so you can get at that menu)
Right click on the tool bar area and choose "Customize

Then find the menu to which you wish to add it
Then go to the middle oolumn and select "all actions" / "Message Editor form"

With that open, go about half way down the list and you will see "add a file" with an icon. Just add it and use the arrows to put it where you want it on the menu. (if you want MIME-coded, you can still add the icon on the far right bottom)

I hope that helps