Haroopad seems to register the handler as:
%APPDATA%\Haroo Studio\Haroopad\haroopad.exe %1
This makes some files, when double-clicked, be opened in the 8.3 filename, like "GENERI~1.MD".
It can be easily fixed, if the handler is registered as:
"%APPDATA%\Haroo Studio\Haroopad\haroopad.exe" "%1"
Now my local problem is gone. And I wish this could be fixed in the official release to benefit others. ;-)
Customer support service by UserEcho