Spec change: https://github.com/whatwg/html/commit/688a102b52c7050a6808a05c5d8fe149c0937fd7
This is part of a larger feature for async clipboard APIs; see discussion in https://github.com/w3c/clipboard-apis/issues/33. But it can be implemented independently.
This constructor will also allow the instanciation of a FileList (it currently works on Firefox / Chromium)
Without this API it's impossible to create a FileList from a File array
const data = new DataTransfer() // This is not working on webkit
files.forEach(file => data.items.add(file))