Work around a bug in older DownloadManager versions.

This adds a number to the extension.

Note that for DownloadManager to keep this, the server
needs to send it a mime type it does not recognize. Right
now, it does not recognize application/json so it's okay,
but we'd do well to remove the content/type header from
the server to prevent problems.

Bug: 8467516
Change-Id: Ic484f66ac3f67c36f59f2c0bcb8c7fdeb6e8590d