I think it is dependent on the application used for e-mail. For example, my e-mail client is set for plain text only, so e-mails that are heavily stylized arrive as text with attachments. My application (Powermail) allows me to switch to viewing as HTML either internally or in a browser.
On a Mac since 1984.
Currently: 27" iMacs, Macbook Air, macOS 10.14.x,; iPhones, iPods and iPads galore!