Because FileTransfer already is able to do encryption/decryption, I wonder if it is necessary Data Extract in combination with it, in such 2 use case scenarios:
- posting encrypted list to MC
- file is encrypted and put on FTP
If you had your file in MC SFTP already, you can use "File Transfer" activity (you have to configure it property though) followed by an import one for instance (this could be automated by leveraging Automation Studio).
Data Extract is not required in the scenario you have described.