The Untertow DefaultServlet exhibits some odd behaviour when it has been mapped to a subdirectory.
To see this behaviour, build the WAR file from the sample project at https://github.com/mcasperson/wildfly-default-servlet-bug. Deploy to WildFly 10 CR5, and open the link http://localhost:8080/wildfly-default-servlet-bug/static (note no slash on the end of the URL). You'll see the expected directory listing.
Now open http://localhost:8080/wildfly-default-servlet-bug/static/ (note the slash on the end). You'll actually now be browsing the root directory of the application.
Now open http://localhost:8080/wildfly-default-servlet-bug/static/static/data.xml (note the double static in the URL). You'll be able to open the XML file. This is not what I would have expected.
Now open http://localhost:8080/wildfly-default-servlet-bug/static/data.xml. You'll get a 404. I would have expected to be able to open the file using this URL.