[DownloadURL] item name for lists and validating Blob transfer URLs

[slightlyoff]

Per a recent conversation, a quick note here to make sure that we expand the Explainer and example code to cover considered alternatives for naming of DownloadURL in the case of the JSON-formatted list, as the serialization presented here is not a valid URL, which I anticipate creating friction in spec conversations and in testing.

We should also validate and document the behaviour of the system in the face of Blob URLs and URLs built from File objects using getObjectURL().