The above would make any entries provided by the mime-support package (as found in the /usr/lib/mime/packages directory) take priority over everything else. The gv package will be used over anything else when it comes to postscript documents. Netscape will be used for any html documents and less will be used for any remaining text documents. However, since neither netscape or less provide for editing documents, any edit or compose actions will fall through to the emacs rules.
After modifying this file, be sure to run /usr/sbin/update-mime (as root) to propagate the changes into the /etc/mailcap file.
Remember that this files takes package names and not executable names. If you want to define rules that reference specific programs, the best way is to include them in ~/.mailcap or the user section of the /etc/mailcap file.
Also, I would like to add the ability to specify certain actions in the rules. For example, if netscape were to have an edit rule but I wanted to use emacs for editing/creating html documents, I could place a line like
before the netscape entry. The update-mime program would then spit out entries such that netscape view rule comes before the emacs view rule but have the netscape edit rule comes after the emacs edit rule.