That is not a supported use-case and it would deliver arguably no benefit as opposed to directly uploading your files. When you use Symphony REST APIs to send a message with an attachment, the message and file data are transmitted and stored encrypted on the Symphony Pod in the cloud. Customers do not get access to the encrypted version of the payloads and there are no APIs to retrieve that. The REST APIs need to work in tandem with the Symphony Clients for any given workflow such that the encryption and decryption process are transparent to both the developer/bot and end users.

Moreover, you will only achieve the highest level of security if you have an enterprise setup comprising an on-premise Key Manager and API Agent. That will prevent any unencrypted traffic from being transmitted beyond your internal network, but that will cost more on a licensing basis as well as infrastructure costs, which might not make sense in your scenario.