Getter method for the file types that this instance
of the serarch engine is configured to use by default
if the calling client doesn't pass on this argument
in the calls.
This search engine can either be set up in advance with all
the configurations, or the execute metods can be called with
the relevant configurations as arguments, thus overriding
any defaults set already.
From the main servlet handling the searches you instantiate
the SearchEngine in the overridden
GenericServlet.init() method by passing
SearchEngineConfig object to this constructor.
Setter method for the file types that this instance
of the serarch engine going to use by default if the
calling client doesn't pass on this argument in the
calls.