Explain more clearly how to use rest/db/file

[timabell]

This adds an example curl call and explains the parameters more clearly.

It's not immediately obvious how to use this endpoint from the existing docs. Hopefully this will help others figure it out. Reading the forums I don't think I was the only person confused by this page.

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🔨 Latest commit	383dfc2d4eb119f0b92ffcf6f152aacdf092fb8b

[timabell]

Thanks for all the suggested improvements to my patch, I've applied them all now