I'm currently having IDS-K9-sp-5.0-2.2 and getting "The filename ... is not a valid upgrade file type." each time when I upgrade with ftp to download either "IPS-K9-5.1-6-E1.pkg" or "IPS-K9-cd-1.1-a-5.1-6-E1.iso". What is the correct upgrade system image that I should upload?
Have you tried upgrading to 5.1(1) first, then 5.1(5)? If you are upgrading from the CLI, the .pkg file is what you want. The .iso is used to create a bootable CD (some sensors may not have a CDROM drive) to recover the sensor.