From rejetto wiki

[edit] What is a MIME type?

A MIME type is effectively information describing the kind of document retrieved from the web server. Since a web server can furnish many types of documents, the browser needs a way to know the specific document type in order to display it properly.

[edit] Is not the file extension enough?

Sometimes file extension is not enough. For example, In the web environment, when a PHP script generates a jpeg image, you get an URL with a .php extension. However, the browser knows that the content represents a jpeg image because it gets this information in the http header as a MIME type.