The program is designed for placing all the files on the disk, not for placing only a single file at a specific location. It's possible, but a bit awkward. You are on the right track with ExcludeFiles and AtPercentageOfVolume. Here is an example script, modify as needed:
# Exclude the files that you do NOT want to move to the end of the disk. */
ExcludeFiles(All and not(FileName("*.avi")))
# Analyze the volume and move the not-excluded files to the end of the disk.
p.s. At the moment this script will be extremely slow because it treats the files that are selected with ExcludeFiles() as unmovable files, and just after analysis it tests if the unmovable files are still unmovable. I am working on it, this script will be faster in the future.