FM Server 12 will generate a thumbnail of the size needed for the specific field on the specific layout.
Oleg's demo screen was to show the realtive file sizes which might be required -- highlighting the advantage of sending only the optimized thumbnail over the network, as opposed to how it worked in earlier versions where the larger file data was sent to the client for rendering.
Is it possible to export the thumbnail rather than the full file?
This is possible, and I have done it with an intermediary step involving a second container field:
Using the new Container Functions, set the content of a second container to actually contain a file of the thumbnail show in the first container -- GetThumbnail ( fully documented in Help ). Then export the content of that second container, and you will get the actual thumbnail as an image file outside of FileMaker.