Whole-file SHA-1, or null for multipart uploads.
Content-Type the file was uploaded with.
B2 file ID.
Custom X-Bz-Info-* headers attached at upload time.
B2 file name (the key).
Byte size of the file (from Content-Length).
B2-side upload timestamp in milliseconds since the epoch.
Result of headCommand: metadata read from a HEAD request, no body.