I have made some further tests. It looks like it generates a date near the start of JBoss and does not use the information contained within the WAR.
It can be reproduced by writing a small java app which does the following:
URL url = new URL("http://127.0.0.1:8080/yourapp/yourFile.html");
URLConnection con = url.openConnection();
A closer look at the modification date shows that it is not the date from the WAR. If you restart JBoss and execute the app again, you get a new modification date.
The solution to my problem was to switch to Tomcat.
Jetty produces the (for my opinion) wrong result. Is this behaviour desired?