Content-Type in Detail — How Email Knows What It Contains

One header decides whether your email client shows plain text, renders HTML, displays an inline image, or offers a file for download. That header is Content-Type, and Chapter 7 is entirely about how it makes that call.

It starts with the basic type/subtype pair — text/plain, text/html, image/jpeg — and the charset parameter, which matters more than people expect: a mismatch is the actual cause behind most “weird symbols instead of letters” complaints.

From there it moves into multipart types, specifically the distinction between multipart/alternative and multipart/mixed — genuinely different tools, not variations on a theme. Alternative says “here are several versions of the same content, pick the best one you can render.” Mixed says “here are separate, different pieces,” which is how an email carries both a message body and a PDF attachment.

Content-Disposition gets its own section too, because it’s the header actually responsible for the inline-versus-attachment decision. Pairs directly with Chapter 8, which goes deeper into how those parts get encoded for transport.