I am trying to use the online demo and even my own local installation but, in both I am facing the same issue.
When I go to newsletter module and try to upload an image from newsletter add template section. It does not allow me to drag and drop and upload the image.
Also, if I choose by browsing the image for a second some other window appears and immediately disappears. This is happening consistently.
I did use different size of images and different types like PNG, JPG but this is reproducible all the time. Can you please help me with the same?