Tip: "portfolio" is the name in the UI. Internally these are called "collections". It is "collection" you need to reference in the documentation. For instance, you can in JavaScript get the list of files ("fields") and present a UI to the user to select one. The user probably needs to be involved in this, so you may just need to show a message telling them what to do; whatever default behaviour you pick is unlikely to please everyone. (By the way, I didn't find a way to set the current open field, but I didn't look very hard).
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