Media type / multipart
multipart/alternative
The same content in several representations, best last — the mechanism behind an email carrying both plain text and HTML.
Served inline: Browsers have no renderer for this, so it downloads even without a Content-Disposition header. That makes it the quiet default for anything you do not want opened in place.
Browser behaviour
Downloads
Charset
no charset
Compression
Already compressed
Send it like this
Content-Type: multipart/alternativeA binary payload has no character encoding. A charset parameter here is meaningless and occasionally confuses strict parsers.
Handling verdict
Browsers have no renderer for this, so it downloads even without a Content-Disposition header. That makes it the quiet default for anything you do not want opened in place.
A binary payload has no character encoding. A charset parameter here is meaningless and occasionally confuses strict parsers.
The format compresses internally. Another transport encoding costs CPU on both ends and typically changes the size by less than a percent — sometimes upward.
Registered with IANA through a public review process, so the name is stable and every implementation can rely on it meaning the same thing.
Anatomy of the name
RFC 6838Top-level type
multipart
Multipart
Subtype
alternative
Registered in the standards tree.
Structured syntax
none
No suffix, so the payload format is defined entirely by the subtype itself.
Parameters
boundary
Defined for this type in addition to any charset rule above.
What trips people up
1 note- Order is significant: the client picks the last part it can render, so the plain-text alternative goes first.
Response headers
Content-Type: multipart/alternative
X-Content-Type-Options: nosniff
Content-Disposition: attachment; filename="example"nosniff stops the browser second-guessing the type you declared, which is what makes the rest of this reliable. Content-Disposition: attachment names the saved file and removes any doubt about rendering.
Server configuration
route headernginx
# multipart/alternative has no file extension — set it on the route instead
add_header Content-Type "multipart/alternative";Apache
# multipart/alternative has no file extension — set it on the handler instead
Header set Content-Type "multipart/alternative"Caddy
header Content-Type "multipart/alternative"