A multi-purpose internet mail extension (MIME) is an expansion of an email system to include more detailed aspects such as the use of attachments, whether these are audio, images or videos for instance.
A MIME comes with more options for text character sets rather than being restricted to just ASCII, the American Standard Code for Internet Exchange.
MIME was set up for mail systems, and creating more options when sending a message to a recipient. However, it has also become increasingly used for HTTP, Hypertext Transfer Protocol, when communicating data online.
A MIME header is simply included at the start of a web transmission when using on the web.