Currently, attached images are displayed in the email. The way they are display can hardly be called a thumbnail preview. They look as if they were added inline. Moreover, these previews are very large, so they obfuscate the layout of the email, while composing or reading.
It would be nicer and less confusing if attached images could get a preview option similar to the way pdf attachments are currently previewed: in a small box that shows the name of the attachment + a sample of its content.
(Originally posted by asparc on GitHub.)