if part.Error != "" || !part.Exists {
			return oi, InvalidPart{
				PartNumber: partID,
			}
		}

		var pfi FileInfo
		_, err := pfi.UnmarshalMsg(part.Data)
		if err != nil {
			// Maybe crash or similar.
			bugLogIf(ctx, err)
			return oi, InvalidPart{
				PartNumber: partID,
			}
		}

		partI := pfi.Parts[0]
		partNumber := partI.Number
		if partID != partNumber {