I would do multiple tabs in the dialog like:
- info that is relevant for the user (who read it, how big the message / attachment is, and when it was sent / received, possibly also delivery errors)
- (if bjoern and others agree) reactions
- advanced (message id, imap ids, hop info and the other technical info)
attachment properties (dimensions, duration, mimetype) could be also in advanced for now, there we don’t need to style them, but in the future they might get onto the first page.