When drag-and-drop an image into the text area, it becomes an attachment
However, you can not place it in the message, because the button is missing to do so.
Adding a second image by drag and drop into the text area, it becomes attachment too, but now the button apears for this attachment to place inline
Then I dropped 5 images at once to the already holding 2 images text area, and wonder:
The first made attachment, still no button to place inline, but now the 'place inline' button also disappeared for the second made attachment
The 5 attachments dropped in the text area, had the button to place them inline
however, not quit sure if it is theme related or a bug, have not tested that one yet.