The chunks for the preview receive a higher priority, but aMule doesn't refuse to download other chunks meanwhile.
Currently aMule give higher priority to the first and the last chunk. And if the last chunk is small enough (1/3), aMule give high priority also to the next-to-last.
But this strategy doesn't work always. It happened to me to wait until 90% of a 1,5GB file, before to receive the next-to-last chunk and to be able to get the preview.
Then I suggest to give always high priority to the next-to-last chunk too.