Per discussion today, we want to set a new standard that

- When attaching files, Testlog should check if current session has serial number set or not. If no any serial numbers (all_serial_numbers), raise failure.

- When attaching files, the key should contain "the expected file name" without information that TestLog can provide. For example, it should not contain serial number or time stamp. Those information can be appended in download script which should rename hash-based file name from GCS to a descriptive name for example "%{serial_number}.%{time_stamp}.%{att_id}".

- Make an easy to use download script.