HTML documents are accessed by the web browser by sending the request to the web server regarding the documents. 
MIME extension is used to deliver the messages from the web browser to server and vice versa. 
The MIME type needs to be stored on the server to use the formats like images, sounds, word processed documents and others. 
The association is being made using the filename and the extension through which it needs to be attached and sent over to the Web. 
As the request is being given in HTML document in the browser the extension gets associated with the browser and it is being rendered into a browser.   _{Your Comment}