Currently there isn't a way to post files to ATD using DXL (they can be posted via the ATD REST interface).
However, there will be an ATD-specific OpenDXL client library released in the future (similar to the current TIE and MAR releases). There was some discussion of possibly including a service wrapper to complement the client. One of the features of the wrapper could be the ability to post files to ATD over DXL. We will have more specifics to share in the near future.