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The media preview shows different qualities of the asset. Out of the box it only shows previews from CM. You can add your CD servers to the configuration parameter DFS.Administration.contentDelivery.baseUrls in App_Config\Include\damforsitecore\DFS.Settings.config. This is useful when you wants to verify that the image is the same on CM and CD servers. If you for example replace an asset in Digizuite and you never publish the changes, the image on CM and CD will be different if it is cached on CD.
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