There is no support currently for html files, but how hard this is depends on what you want from to do.
To merely upload, store and view the raw html, then add the text/html mime type to your configuration files, using the same rules that text/xml uses. If you want it to render a workable site using a webarchive or attaching sidecar files, then that will require development work, which you might be able to request and fund it's development.