A generic MIME type, I assume, would be the default for files that don't match any other MIME types. Depending on what you set it to, the file will be treated different ways. For example on a webserver if you set the default MIME type to binary/octet-stream or whatever it is, browsers will prompt to download. If you set it to text/plain, they'll display it as plain text in the browser, even if it really is a jpeg or a zip file or whatever.