DesignManager - programmatically add PDF attachment from CAS?


I'm trying to configure an application in DesignManager, that casually should get a certain PDF attachment. For convenience the PDF should be stored in CAS so it can be replaced on the fly, once necessary without deploying a new application.

I created a "Placeholder Document" with a dummy page, as the documentation suggests.
I created a placeholder variable of type PDF
I even can use this "page" in Empower to manually upload a local PDF

What I'm missing is how to automatically preset the variable with a CAS stored file.

The "placeholder" register allows for the option "CAS resource" but I found no place to actually point it to a certain CAS resource. And I found no hint in the docs either.

Bonus question: Why Can I pick an image from CAS with ResourceBrowser, but no PDF attachment?