Send correct MIME type instead of text/plain

When another file (e.g. a CSS stylesheet) is referenced from an HTML file, `impatient-mode` serves it with a hardcoded `Content-Type: text/plain`. This causes problems in browsers; for example, Firefox ignores CSS files that aren't sent as `text/css`.

This commit makes `impatient-mode` determine the MIME type to send based on the extension of the requested file.