I have an Zyxel NSA-200 Plus NAS at home. Its age taken aside, a pretty solid piece of hardware. If you are wondering why your attempts of a firmware upgrade fail with the following message, here is the solution.
"Invalid firmware file."
From its original firmware version V3.00(AFG.0) you have to sequentially upgrade the next point release. Namely, this is V3.10(AFG.0) and V3.12 (thanks to pressworthly for this information) before you can upgrade freely. You can find all firmware versions for the NSA-220 Plus here. Bear in mind that the first two upgrades have to be done using a USB stick. I used the procedure described in this blog post (german).