Tell you what, go into your mail client and view the original source of any email with inline images. In Gmail it’s called “View Original”. Not sure about other mail clients, but if in doubt you could export it to a file and open that file in your favorite text editor.

Let me know what you find.

Edit: also do it for an email with what you consider to have an attachment. Doesn’t matter what kind; any attachment will suffice for this experiment.