In Windows when I double click on any *.mm file FreeMind is not launched with the correct file loaded. This seem to be entirly due to the included FreeMind.exe. As mentioned in your documentation
Freemind - Known Bugs & Workarounds
Launching FreeMind by launching a FreeMind map will at least in Windows cause FreeMind to work improperly: When you call the Documentation from the Help menu, the corresponding file will not be found.
You should launch FreeMind and open your map using the file menu.
This is quiet funny because in the Source code the Readme for the windows launcher says
"This launcher requires no more development for it does quite everything one would
expect from a java launcher:"
To fix this problem I spent 10 minutes creating a JSmooth (https://sourceforge.net/projects/jsmooth/) Config file (which I will happily send to any one).
I then compile it and I can open FreeMind with the newly created FreeMind.exe and even double click to open *.mm files.
Would the project team mind updating your windows launcher, or consider switching to a JSmooth build for it.