Link to the ROS2 Message spec documentation for reference: https://docs.ros.org/en/rolling/Concepts/Basic/About-Interfaces.html#messages

Thanks for the pull request and reference. One change requested is to also add the file extensions into the appropriate place in main_test.go For the # comment style that should be https://github.com/google/addlicense/blob/master/main_test.go#L347

I would be careful about inferring a file's type based on a .msg extension. Microsoft Outlook uses .msg for emails and other data and as a shorthand for "message" it seems likely for people to use it for data in custom formats. (Similarly, there's no single format for a .log file.) It looks like there are multiple uses of .action as an extension as well, including macOS Automator.

Handling generic extensions like these is a good use case for having an addlicense config file. A standard ROS2 addlicense config could be part of a project template without "claiming" these extensions for everyone.