The .bin file needs to have "SCE...." header. You can easily see in a hex editor if that is true. scetool, or "open scetool". Both are open source now on github and can be compiled to any architecture. Download the latest source code from github and compile it on your machine. Then try 'scetool -i <filename>' or 'scetool -d <filename>'.
Everybody uses 'scetool' these days, since 4.2.X firmwares the previous set of older generation NPDRM tools (from 3.55 & Geohot days) no longer work for everything.