The UUENCODE Attachment Format

One of the earliest methods of adding attachments to a message was to append the attachment to the US-ASCII body text using the Uuencode encoding mechanism. Each attachment was prefixed with a preamble that listed the file name and the encoding termination string. Multiple attachments were appended in sequence, and separated by a blank line. In such a format, the body of the message was broken into two or more parts:

To: "Some One" <someone@microsoft.com>
From: "Another" <another@microsoft.com>
Subject: My latest draft.

I've attached my latest draft.
Later,
Some One

begin 644 mydraft.doc
AAAAAAAAAAFDFAAAAFAFAF....

In this format, the body of the message consists of two parts: the text message and the attachment. If other attachments were added, they were appended on the end of the body.