By now only html files are equipped with the charset option afterwads, but not CSS or JS files.
org.apache.wicket.request.resource.AbstractResource#setResponseHeaders() does take the textEncoding into account,
but there is no way to set it from the outside without introducing new classes by copy/pasting existing Resources and adapt them.
The only resource which is able to modify the content-type through the constructor is the TextTemplateResource.
It's a hack, but this string then used for the content-type header.
A separate charset parameter would be better in my opinion.