MIME Base64 (RFC 2045)

MIME (Multipurpose Internet Mail Extensions) introduced Base64 as a Content-Transfer-Encoding so that e-mail, originally limited to short lines of 7-bit ASCII, could carry images, documents and non-English text.

Rules

• Same alphabet and padding as standard Base64.
• Encoded lines must be no more than 76 characters, separated by CRLF (\r\n).
• Decoders must ignore any character not in the alphabet (such as line breaks and spaces), which makes MIME decoding lenient.

Example e-mail part

Content-Type: image/png; name="dot.png"
Content-Disposition: attachment; filename="dot.png"
Content-Transfer-Encoding: base64

iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6
kgAAAABJRU5ErkJggg==

Encoded words in headers

Subjects and names with non-ASCII characters use RFC 2047 “encoded words”:

Subject: =?UTF-8?B?R3LDvMOfZQ==?=

B means Base64; the value decodes to Grüße.

Produce MIME-formatted output by choosing the “MIME” standard in the text encoder or normalization tool.