Cannot assign 'Open With' to a specific application using Mountain Lion
Have you tried rebuilding the Launch Services database by running /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -seed?

If that doesn't work, you could also try defaults write com.apple.LaunchServices LSHandlers -array-add '{LSHandlerContentType=com.netscape.javascript-source;LSHandlerRoleAll=com.panic.Coda;}'. Applying the changes requires restarting (logging out and back in isn't enough).