Welcome files that correspond to a servlet-mapping are ignored. As an example, add a welcome file "index.jsf" and map "*.jsf" to the faces servlet. The welcome file will not be served when navigating to the folder.
This is not in accordance with the Servlet 3.0 specification (Section 10.10):
... the Web server MUST again append each welcome file in the order specified in the deployment descriptor to the partial request and check if a servlet is mapped to that request URI.
As an example of this in practice, refer to one of the many stackoverflow questions on the subject: