On Windows, the FreeMind installer offers to associate its .mm files with the application in Windows Explorer. On KDE however, you have to do this yourself. I happened to put the missing pieces together on how to do this when looking over Beagle index logs that revealed the FreeMind MIME type to me. The steps are:
- Run kcontrol (KDE Control Center).
- Under KDE Components, choose File Associations.
- Click Add. Change the group to text, and enter x-troff-mm for the type name.
- Under Filename Patterns, enter *.mm. Fill in the description as FreeMind Mind Map.
- Under Application Preference Order, click Add, and browse to the FreeMind application (on mine, it installed under Office; you can probably just enter /usr/bin/freemind).
- Click Apply and you're all done.