The main thing that AirWave uses to determine firmware for a device is the Instant Firmware version.
Let's breakdown the Firmware file:
ArubaInstant_Orion_6.4.2.3-4.1.2.1_49445
Orion determines which APs can use the firmware (in this case IAP 92/93,104/105,175,RAP3WN/P)
The 6.4.2.3 is the AOS code base (negligable for Instant)
The 4.1.2.1 is the portion AirWave cares about.
And the 49445 is the specific build number (for internal tracking)
So if you have AP225 and AP105, you'd need both Centaurus and Orion images. If you do not want to download and upload both images, you can use the Desired Version portion to get the images from the image server (activate.arubanetworks.com).
Under Groups -> Firmware
If you choose a firmware image under Image Server, then AMP will contact activate and grab all the needed images for the devices in that group (as long as your AMP can reach the internet, more specifically the activate server).
The drop down list will only list the version, and not the compatible device version. So if you're manually uploading, then you'll need to make sure you upload all copies of the image needed.