How can I generate file preview thumbnails in my desktop application?

I am developing a sort of file browser on the Mac, and I'm not sure how I can dynamically generate thumbnail previews of files. I would like to do this not just for images开发者_如何学JAVA, but for common document formats, html, videos, etc., just like in Finder. Is there a built-in API for this, or would I have to search for an external library?

Also, how would I go about doing this on Windows?

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i use geektool on mac to generate thumbnal previews for images and files, but i don't know witch library use