How do attachments work in email?

Attachments are files included in email messages using MIME encoding. Since email was designed for text, binary files must be converted to text-safe formats for transmission.

How attachments are structured:

The email uses multipart/mixed MIME type

Each attachment is a separate MIME part

Binary content is encoded using Base64 (converts binary to ASCII characters)

Content-Disposition: attachment indicates it should download rather than display inline

Example headers for an attachment:

Content-Type: application/pdf; name="document.pdf"

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename="document.pdf"

[Base64 encoded file content]

Size impact: Base64 encoding increases file size by approximately 33%. A 1MB file becomes ~1.33MB in the email. Combined with message overhead, attachments significantly increase email size.

Inline vs attachment: Content-Disposition: inline attempts to display the content within the message (common for images). attachment prompts download.

Most email providers limit attachment sizes (typically 10-25MB). For larger files, send links to cloud storage instead.