When it comes to previews, the server does convert them from time to time to meet the size/dimensions of what is required.
In order to bypass this, using the program Windows Movie Maker, create a 9 – 20 second preview. Once it has been chosen, select > File > Save Movie > for Computer and this will take the necessary dimensions to avoid any converting.
You can use any program of your choice , as long as the Frame rate is 29 frames per second, also 854 width by 480 height.
Hopefully this helps. If you have any other questions let us know.