Attachment
A file sent inside an email as its own MIME part, usually base64-encoded and marked Content-Disposition: attachment with a file name.
What it means
An attachment is a part of a multipart/mixed message with its own Content-Type (application/pdf, image/png and so on), a transfer encoding, normally base64, and a disposition telling the mail program to offer it as a file. Base64 adds about a third to its size, which counts toward the receiving server's size limit.
Receivers inspect attachments closely. Gmail blocks messages carrying executable and script file types such as .exe, .js, .bat and .ps1, and also limits how many attachments one message may have. A link to a hosted file avoids size limits, though the linked domain's reputation then becomes part of the message.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2183: Communicating Presentation Information in Internet Messages: The Content-Disposition Header Fieldrfc-editor.org
- RFC 2045, section 6.8: Base64 Content-Transfer-Encodingrfc-editor.org
- Google, Gmail Help: File types blocked in Gmailsupport.google.com
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes