File object fields
Audio files are represented as JSON objects which have the following keys:
Unique file id
It is unique only within a scope of this call instance.
Date/time when audio recording for this particular file has been started.
Format is ISO8601 with timezone, for example,
|stop_time||datetime||Date/time when audio recording for this particular file has been stopped|
|file_size||integer||Size of audio file in bytes|
|file_path||string||Location of audio file on disk.|
|watermark||string||SHA1 hash of audio file content. It is used to validate data integrity|