We use automation to create a new issue for dev-team once the design is ready.
However I can't find a way to copy Figma plugin attachment from the parent issue. It seems like Figma plugin doesn't match any typical field like "customfield" etc.
You may want to check that marketplace vendor's documentation to see if they support that with automation. I suggest checking here for the specific product you are using, and then check their "support" tab: https://marketplace.atlassian.com/search?hosting=cloud&query=figma
While waiting for the vendor's response, you could also use the technique described in this article to learn if there is a smart value for the field. Basically you find an example issue with the data, use a browser to call the REST API, and the search for your field/data in the results.
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