In E-mail: A file that is included with the E-mail message, typically not containing plain text.

In Internet (SMTP) E-mail, there are two widely supported ways to use attachments: uuencode (now getting rare) and MIME (these days the preferred format - for non-textual formats, MIME uses base64 encoding).

These days, the file attachments are considered a small security threat; Many people use Microsoft Word documents as their "E-mail format" and this may lead to a virus epidemy. Some viruses even sent executable attachments that look like normal file attachments to untrained eye (see ILOVEYOU). Of course, there's no such problem with image files and such things that do not have executable content.