[DownloadURL-list] Try to make this uniform with the rest of drag&drop

[jyasskin]

@joone

https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/DownloadURL-list/explainer.md proposes to extend the Chromium-only DownloadURL system to become a way to download multiple files at once. @avidrissman noticed that it would be more uniform with the rest of the platform to treat these as File elements of dataTransfer.items.

Could you either switch to that approach, or explain why it's a bad idea in an Alternatives Considered section?

cc/ @dandclark @slightlyoff