Unlike Windows, Linux does not have a core facility that
launches applications based on a file's MIMEtype. Such
capabilities exist in various places - such as in the file
managers for the desktops - but I know of no command
you could reliably use in all Linux environments to launch an
application from a file.
One possible solution might be to
launch the file in Netscape itself, with the
AppletContext.showDocument() method. Netscape would
presumably "show" the document in some way that makes
sense for the document's MIMEtype.