Best way to attach supplementary material?

What is the best way to attach supplementary material to an item, and easily open either the main paper or the supplementary document?

I'm sure Zotero must have an easy way to do this, as it does for everything, but I have trouble finding it. I moved back to Zotero from Mendeley only a few months ago, and my library is still in shambles. I have several items with more than one PDF included, but I can't figure out how to control which of these will open upon double-clicking the item. In some cases it seems to treat the supplementary document as the primary. How can I fix this?
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