Undertow incorrectly resolves the MIME type for resources that are usually directories (i.e. they have no file extension) in ServerContextImpl:

 @Override
    public String getMimeType(final String file) {
        if(file == null) {
            return null;
        }
        String lower = file.toLowerCase(Locale.ENGLISH);
        int pos = lower.lastIndexOf('.');
        if (pos == -1) {
            return deployment.getMimeExtensionMappings().get(lower);
        }
        return deployment.getMimeExtensionMappings().get(lower.substring(pos + 1));
    }

Even for a directoty named "foo.css" I wouldn't expect the server to return such a MIME type. IMHO, the implemenation should check if the resource is a directory and return null in this case. It also makes sense to return null when file has no extension (instead of treating the file name as a one).