Fail to flash OpenWrt Attitude Adjustment built with FMK

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Download this precompiled OpenWrt 12.09-rc1 image: 
http://ltl.tkk.fi/~malaakso/misc/openwrt-lantiq-danube-ARV7510PW-squashfs-with-a
th5k.image
2. Extract it
3. Build it
4. Try to flash it via OpenWrt WEB-UI

What is the expected output? What do you see instead?
Expecting the firmware to work. Instead, I'm seeing "The uploaded image file 
does not contain a supported format. Make sure that you choose the generic 
image format for your platform."

What version of the product are you using? On what operating system?
Trunk. Ubuntu 13.04 i686.

Original issue reported on code.google.com by [email protected] on 11 Oct 2013 at 7:56