To register a new file extension with an application use the following defaults command.
Replace PUT_FILE_EXTENSION_HERE_WITHOUT_PERIOD with the file extension i.e. txt.
Replace org.category.program with the com/org name of your program i.e. com.apple.itunes.
$ defaults write com.apple.LaunchServices LSHandlers -array-add \
Once you have added the file extension to the launch services, you must restart the launch services deamon so it will re-read the configuration file.
You can either run the command below to restart launch services, or simply restart your computer. Loging in/loging out also might do it but I haven't tried.
$ /System/Library/Frameworks/CoreServices.framework/Versions/A/Framework/LaunchServices.framework/Versions/A/Support/lsregister -kill -domain local -domain system -domain user