OS X mail has a nasty habit of alway. I mean always embedding images. Mail doesn’t seem to know what you mean by attachement or even “Send windows friendly attachment” it’s not friendly at all. The problem is html formatting most mail clients want things to look pretty and put the image into the email as if it’s part of the email. What happens when you need the picture to be an attachment? Well there is one way to do it. Simply right click the image and select “View as icon” that is it. There is no other way that doesn’t involve a third party plugin. Send windows friendly attachement doesn’t work neither does the option to place attachements at the end of the message. If you want to send an actual attachement to a windows user then you need to right click the image and select view as icon. 

Note: two finger tap works as a “right click” or secondary click if not you can control+click the image.