Doesn't work in web workers

[temeddix]

On wasm32 target, the file dialog doesn't work when exeucted from a web worker, not the main thread.

I think this problem needs a hand, because running Rust code in web workers is a common situation. Though window is not defined in web workers, there is still FileReader API that it is still possible to be used.

https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Functions_and_classes_available_to_workers
https://developer.mozilla.org/en-US/docs/Web/API/FileReader