Progress for attachments uploading

[slav0nic]

Will be nice have some basic functionality for get file uploading progress
Usually it implemented via XMLHttpRequest progress event, but kinto based on fetch() that still doesn't have this functionality.
so, will be nice have progress event in client.events

[dstaley]

So as you mentioned, it doesn't look like fetch emits any sort of progress indicators. Right now there's no plans to switch away from fetch, so I don't think we really have a path forward to implementing this. That being said, I do want to keep this issue open in case the situation changes.