So the new custom firmware thing was implemented in Feb 2020.
Is no one actually using it? Because it does not work unless you make your custom firmware file name match what EPM uses for non-custom firmware.
The EPM Custom Firmware Management wiki does not list this as requirement.
I will go open a commercial support case (edit: I did #997357), and I expect it to get closed to No issue after someone updates the wiki. That is what happened last time I had an issue were something did not work like the wiki said. It is apparently much cheaper to piss people off than fix a bug.
Anyway. Uploaded 2 firmwares. The T40G firmware labeled as T40P because the G is not supported. And then the CP920 because while the model is supported, Sangoma has never, for years and years, put the firmware in an EPM firmware bundle.
But when I rebuild the template, it writes out the default rom names that it uses internally. Not the filename of the uploaded file.
Edit: When you make a custom firmware, the slot (2 in my case) is a symlink to the folder with the uploaded files.