The answer is, nothing will clear that cache apart from one App that I was tought a while ago.
Dropbox won't do it. And no seeing in the iPad or iPhone will do it.
Get - iFunBox. It's free. You install it on your PC. Connect device. Ensure iTunes is running. Then open iFunBox, and Managing App Data.
Show All.
Then click the app (ie Dropbox) and then go to Library. Then Select Cache, and delete.
It won't delete the folder, but you will see the cache build up and it will delete it all.
Works great for Wallpaper apps too, that accumulate quickly if you like changing those. And specially Spotify which doesn't really "stream" it actually (I think) stores the music cache on your device, so builds up pretty fast.