You can't upgrade to Jellybean. The device doesn't have enough RAM or processing power to run newer versions of Android. If you want to try Gingerbread you can install a cyanogenmod based ROM, see http://dev.openaos.org/
The cyanogenmod gingerbread ROM is pretty decent but you lose the Archos customised video player and hardware video acceleration and the Archos File Manager and its built-in browsing of upnp and windows shares (AKA smb/samba/cifs). But you can dual boot regular Archos firmware and cyanogenmod so maybe this would let you do the stuff you want.
Bear in mind that these gen8 devices came out in 2010. They simply aren't able to run the newest versions of Android, mostly because 256MB RAM is too little but also a single core 1GHz CPU is very low end now. These days a typical bargain basement generic Android tablet (the £50 - £80 devices) has a dual core CPU, multi core graphics chip and 1GB of RAM or more and people complain that these are slow. The gen8 devices are archaic in comparison but still work pretty well with Froyo.