Content-Disposition
The MIME header (RFC 2183) that suggests whether a part should be shown inline in the message or offered as an attachment, and under what file name.
What it means
The two values are inline, for a part meant to be displayed as part of the message, and attachment, for a part the reader saves or opens separately; a filename parameter proposes a name. The header is optional, and RFC 2183 says that without it the mail program may present the part however it sees fit.
For a predictable result, give each file Content-Disposition: attachment with a filename, and reference embedded images through Content-ID and cid: URLs inside a multipart/related structure.
Example
Content-Disposition: attachment; filename="invoice-2026-10.pdf"Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes