When you create a thread and upload attachments and then insert them as thumbnails they always appear on top. Your latest cursor position is ignored. This currently doesn't work on iPhone 5, iOS 6.1.4, other phones, tablets not tested.
I'm fairly sure if you click done and then Click on upload file what you say happens. However if you leave the editor (keyboard) open and click upload a file then the image will be placed where your cursor is.