Opening Freeplane files with double-click

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http://askubuntu.com – I use Freeplane mind-mapping software. It creates .mm files which Nautilus reports as Troff MM input document. I want to be able to open a file with Freeplane by double-clicking it. I have a .desktop file for Freeplane with the mime-type set as application/x-freemind. I added a mime-type for *.mm files as application/x-freemind but this didn't change anything. Right-click Open with… still doesn't show Freeplane. Freeplane is installed in a folder under home simply by unzipping an archive and adding a .desktop file under ~/.local/share/applications pointing to the freeplane.sh script file (HowTos)