Just selecting it with the mouse to drag over to the trash folder momentarily opens the HTML page. The only way I've found to delete a message without opening it is to select it with Control-Click after first selecting another message, then drag them together to the trash folder or select Delete from the menu. So HTML mail has some ugly side effects, and they are largely the same on Linux as they are in Windows. Now let's move on to MIME-based file attachments. In Linux, those who use a graphical e-mail client are used to seeing the attachment as an icon, with menu options to save it to disk, delete it, or open it.
Nothing happens without our deliberate action as a user.