Use XMLHttpRequest to upload files

[taoeffect]

Problem

As @SebinSong mentions here, the current approach to uploading files uses fetch, and this prevents apps from being able to display a progress bar for file uploads.

Solution

Rewrite to use XMLHttpRequest and update the API to support reporting upload progress.

Also give the ability to interrupt the file upload and cancel it via user action.

Consider doing the same for the corresponding fileDownload API.

[corrideat]

Relevant: https://stackoverflow.com/questions/35711724/upload-progress-indicators-for-fetch

TL;DR: It is possible to still use fetch by wrapping the payload inside of a stream, if the browser supports that. When this is implemented, see which approach (stream or XHR) is best and go with that, or maybe both (streams with an XHR fallback)