how about sharing files?

[jimmywarting]

One must register a service worker and register a fetch handler that can take care of them...

if a share polyfill sees that example.com have registered as a share target but don't have the service worker installed, the share polyfill must then create a hidden iframe to something like example.com/install-sw.html to pre install it and then create a POST with those files

https://web.dev/web-share-target/#accepting-files