This required checking the file size of every file (including auxiliary
snapshot files) in every attachment to be uploaded, and it was an
estimate anyway, because snapshots are compressed, so we kept track of
the compression ratio, but there was no particular reason to think later
files would have the same ratio... In any case, we're not currently
displaying file sync progress in 5.0 anyway, and while we should, we can
probably just show a count of attachments remaining.
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