If you import an encrypted file of subscriber data, you need to configure Field-Level Encryption in the import file activity. This allows you to specify which fields are encrypted and how to decrypt them. The Manage Files in File Transfer activity is used to move files from one location to another, not to import them. The character encoding setting is used to ensure the correct display of characters in different languages, not to decrypt encrypted data. You can learn more about this in the Importing Data into Marketing Cloud module on Trailhead.