The Atari disk image tools don't allow to place contiki.cfg at a fixed location. Therefore the byte offset of the contiki.cfg content isn't constant :-( However using the tool adir with the option -r allows to learn the block number of the contiki.cfg content. So this block number is stored for each Atari disk image in the PHP code (in the array $blk) and translated into the correct byte offset.
This additional application results in three disks for the Apple2, two disks for the C64 and one disk for the C128. Therefore the download page can't get along anymore without some JavaScript adjusting the number of downloadable disks depending on the selected machine. With JavaScript turned off access to non-existent disks (i.e. disk 3 for the C64) results in a successfull download of a zero byte sized file - not exactly great but at least a defined behaviour.