When you have Business Class you can do it with a single click. You can check this documentation for more information (check "Export all boards with Business Class")
From other ways not sure .. I found below tool but did not tried it
The Idea of the downloader is that you get JSON (which contains information about attachments) and you download via the tool from the second link for example (or your own (if you parse URLs) - for me the link is working so not sure what is wrong on your side. Maybe you can try copy paste it or use different browser.
If you had to thrive a new habit during a lockdown, what would it be? Trello
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