Upload the encrypted file to Business Chat.
To upload the encrypted attachment to Business Chat, send a
POST request to the URL returned by the
/pre call (see Authorizing an Attachment for Upload), with the attachment in the body of the request.
Business Chat returns a JSON dictionary containing the
file field. Store the value from this field in the
signature-base64 key in the Attachment dictionary.
When using Server Name Indication (SNI) (RFC 6066), upload the request in a secured TLS connection to the returned upload URL. The TLS client should set the TLS server hostname from the extracted hostname for the upload URL.