Examples

A commented version of the 'html' example from the README

;; match on file names with the 'html' extension
(list "html"
      ;; define a set of approaches to handling files with the 'html' extension
      (list
       ;; the first member of the set
       (list
	;; the 'label' we provide for selecting this option
	"special html launcher"
	;; the corresponding function
        ;; - the function is called with a single argument, the file of interest
        ;; - the function can do whatever you want it to do
	#'(lambda (file)
	    ;; here we invoke the command 'bluefish' with the flag '-n' along with the file name 
	    (dired-launch-call-process-on "bluefish" "-n" file)))
       ))

You could add something like the above to dired-launch-extensions-map using (push (list "html" ...) dired-launch-extensions-map).