Here you go:
https://github.com/pocketbook-free/SDK_481
I tested firmware 5 on my Inkpad, and extensions.cfg worked there, but it just took the first entry in the list for each file type (without giving you an option to choose). I went back to firmware 4, as well.
P.S. I haven't tested the latest SDK, as is. It looks like it was just a file dump from one of the developers at PocketBook. If you try it, let us know if it works. Since my builds of pbimageviewer are based on sergeyvl's Makefile system (rather than cmake), and I need 64-bit tools, I had to create my own new SDK based on the header files and libraries in SDK_481.