REST Endpoint fetchAndLock: result variables does not contain value for type: File

I want to get the File bas64 encoded value of my File Variable when calling the fetchAndLock endpoint.
Unfortunately it is always null, even when passing deserializeValues = true .
its also hard to get the variables instance-id to use the /data endpoint, as the returned executionId is for the Task not the underlying subprocess.

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Can we add a feature to return more informations?