Improve/browser upload example

Honestly, I think saving to localStorage is overkill for an example. We should try to keep the example as minimal as possible. Perhaps something like this: https://github.com/Automattic/wpcom-xhr-request/blob/2b6f1b2e6b70c6d1eeda4ea66769a457aa362008/examples/browser/upload.html#L56-L72