No longer a labs option, file attachments in Google Calendar are going to the stable channel. A feature oriented towards shared events, this can allow you to share important information that can't be captured in a written description. It is live now on the web and the API is ready for use, which means we should see app developers taking advantage in the near future.
You'll see that "add attachment" option when creating a new calendar event, and when clicked you will see something like this:
That should look familiar if you've shared files on the web with any other Google product in the recent past. A nice detail included is that if your event is shared with anyone who doesn't have permission to view the file in Drive, you will be prompted in case you want to change that fact. This will come up if you have been working on internal documents that, at one point, weren't ready to be shared more widely.
Beyond the web rollout which should be fully live today, developers should start using the updated API soon as well.
- Google Apps Blog