key "url_download" not present

[peddie]

The JSON object for a file download is huge and contains tons of internal information, so I'll skip pasting it out of laziness.

url_download and url_private_download are treated as mandatory in File.hs. This matches what the API docs say. I seem to be getting FileShared and FilePublic events that don't contain either of these fields (and have never received a File-anything event that has either).