it seems that https://www.mediawiki.org/wiki/API:Imageinfo doesn't allow for querying the caption and structured data.
https://commons.wikimedia.org/w/api.php?action=query&titles=Image:Commons-logo.svg&prop=imageinfo&iiprop=extmetadata is given as an example
But https://commons.wikimedia.org/wiki/File:Commons_logo.svg has a caption and structured data like "depicts".
It should be possible to query this through the mediawiki api.