Any way to force reset of all cached static files on AppEngine?

It the depends on what cache-control is used. Check in firebug och chrome inspector and see what expiration date are set.

If you've set the cache-control to public you can't affect the control since the files are cache on various proxies and server along the way.

If you use cache-control private you should be able to reset you browser cache and be fine, but as you say sometimes you get the wrong files from Google's production environment. I've had the same problem. The fastest solution is to add a query param to the files loaded.

There is a way to shorten the expiration time for static files cached by your app on Google Cloud.

Head to your Google Cloud Console and open your project. Under the left hamburger menu, head to Storage -> Browser. There you should find at least one Bucket: your-project-name.appspot.com. Under the Lifecycle column, click on the link with respect to your-project-name.appspot.com. Delete any existing rules, since they may conflict with the one you will create now.

Create a new rule by clicking on the 'Add rule' button. For the object conditions, choose only the 'Newer version' option and set it to 1. Don't forget to click on the 'Continue' button. For the action, select 'Delete' and click on the 'Continue' button. Save your new rule.

This new rule will take up to 24 hours to take effect, but at least for my project it took only a few minutes. Once it is up and running, the version of the files being served by your app under your-project-name.appspot.com will always be the latest deployed, solving the problem. Also, if you are routinely editing your static files, you should remove any expiration element from handlers related to those static files and the default_expiration element from the app.yaml file, which will help avoid unintended caching by other servers.