A forensic scientist would testify that they extracted the files from there.

It is exactly the same as when a confidential source gives up an address (for example) the source never gets into evidence.

The whole point of strong encryption is to prevent adversaries (including forensic scientists) from extracting any information without possession of the key.

If the key involves a password that you, a human, have memorized in your squishy pink organ, it's privileged under the Fifth Amendment. (This hasn't been tested in court yet, of course. There's no precedent to fall back on.)