I want to use mutations to upload files too, but I can't find any way to track the progress of the mutation (its upload, similar to the XMLHttpRequest progress event). Afaik relay is using fetch, but fetch doesn't provide any way to track progress (https://github.com/whatwg/fetch/issues/21).
So, what's the best solution? Write a custom network layer which uses XMLHttpRequest? But then again, how to get a reference to the request object back to the caller of Store.commitUpdate
...?