The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform.
The same error is reported for the prebuilt snapshot image as for a self created imagebuilder image.
Observation:
For the same target on a ar71xxx snapshot imagebuilder the build o/p has the following in it:
Building images...
Unable to open feeds configuration at /home/re/openwrt-imagebuilder-ar71xx-generic.Linux-x86_64/scripts/feeds line 48.
25+1 records in
26+0 records out
1703936 bytes (1.7 MB, 1.6 MiB) copied, 0.00195437 s, 872 MB/s
Image Name: MIPS OpenWrt Linux-4.14.96
Created: Wed Jan 30 13:57:40 2019
Image Type: MIPS Linux Kernel Image (lzma compressed)
Data Size: 1683422 Bytes = 1643.97 KiB = 1.61 MiB
Load Address: 80060000
Entry Point: 80060000
Parallel mksquashfs: Using 1 processor
Creating 4.0 filesystem on /home/re/openwrt-imagebuilder-ar71xx-generic.Linux-x86_64/build_dir/target-mips_24kc_musl/linux-ar71xx_generic/root.squashfs, block size 262144.
Pseudo file "/dev" exists in source filesystem "/home/re/openwrt-imagebuilder-ar71xx-generic.Linux-x86_64/build_dir/target-mips_24kc_musl/root-ar71xx/dev".
Ignoring, exclude it (-e/-ef) to override.
[=================================================================================================|] 883/883 100%
For the ath79 generic build for the same target:
Building images...
Unable to open feeds configuration at /home/re/openwrt-imagebuilder-ath79-generic.Linux-x86_64/scripts/feeds line 48.
Parallel mksquashfs: Using 1 processor
Creating 4.0 filesystem on /home/re/openwrt-imagebuilder-ath79-generic.Linux-x86_64/build_dir/target-mips_24kc_musl/linux-ath79_generic/root.squashfs, block size 262144.
Pseudo file "/dev" exists in source filesystem "/home/re/openwrt-imagebuilder-ath79-generic.Linux-x86_64/build_dir/target-mips_24kc_musl/root-ath79/dev".
Ignoring, exclude it (-e/-ef) to override.
[=================================================================================================|] 873/873 100%
The kernel image appears to be missing from the ath79 build.
No, why should I need to?
a tiny snapshot image for ath79 on a wr703n sysupgraded ok.
I've been using imagebuilder updates for a couple of years now and this is the first time i've come across this.
This suggests that the current firmware is for a TL-MR3020, which is on the "tiny" target for ath79 on master, not a TL-WR842N. Do you recall where the image that is presently on the device came from?
Edit: It also appears to be an "old" image on the device, as the current ath79 device is tplink_mr3020-v1