GetActualSize() was heavily relying on o.Parts()
to be non-empty to figure out if the object is multipart or not,
However, we have many indicators of whether an object is multipart
or not.
Blindly assuming that o.Parts == nil is not a multipart, is an
incorrect expectation instead, multipart must be obtained via
- Stored metadata value indicating this is a multipart encrypted object.
- Rely on <meta>-actual-size metadata to get the object's actual size.
This value is preserved for additional reasons such as these.
- ETag != 32 length
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