OK, got one step further.
There's the Advanced Configuration, which can be found under:
Unfortunately it does not say, how more than one expression can be used. It only says "The filter string NCM uses to determine what files to monitor in the firmware repository"
I've experimented with blank, colon, semi-colon, pipe, asterisk only - but none seems to work.
This is a real pity, because it works fine, if I delete the *.bin and replace it with *.cc for example. But this is still not what I want. I have at least three extensions I'd like to use in this context.
I changed the FirmwareRepositorySearchPattern variable to *.* and it pulls in all files in the repository. It's not ideal, but it serves as a workaround for now.