Mime-type

A mime-type or Internet media type is a two-part identifier for file formats on the Internet. The identifiers were originally defined in RFC 2046 for use in email sent through SMTP, but their use has expanded to other protocols such as HTTP, RTP and SIP. These types were called MIME types, and are sometimes referred to as Content-types, after the name of a header in several protocols whose value is such a type. The original name MIME type referred to usage to identify non-ASCII parts of email messages composed using the MIME (Multipurpose Internet Mail Extensions) specification. Without MIME types, email clients would not be able to understand if an attachment file were a graphics file or a spreadsheet etc. and would not be able to handle the attachment appropriately.

Source

 * http://en.wikipedia.org/wiki/Internet_media_type