This is not possible because of security issues via java.

There is a way around it as follows:

1/ Move the html file into the c:\program files\jlogbook\html directory. (e.g. homepage.html)
2/ In the extralinks command, use the http command to access this file. I.e.
extralinks = "<a href='http://<name of machine>:8080/homepage.html'>Intranet</a>"
3/ If the html file references other directories, then these too need to be under the html directory.