DFU - what is "Init packet file" (*.dat) and how to get it?

I now have my app.hex and bootloader is running at nRF51822 and yet, the nRF Toolbox finds "DfuTarg" as expected. However, after selecting my file the UI surprisingly starts to ask about "Init packet" and I just say "NO", because I don't have such a thing anywhere.

Yep, right, the device firmware upload fails after that! Great. Two times it complained about missing init data or something, and two times the error was something else.

So, how to get that stupid .dat file there? And sorry, I am a bit frustrated after yet another fruitless day... complains removed