How to add ffmpeg/avconv command to Nemo actions?

I would like to have a command like this one

ffmpeg -i video.mp4 -vn -acodec copy audio.m4a

into a Nemo action (context menu).

But that would mean using variables instead of real names of files. (Also, preferably, instead of the input extension: I would like a command applicable to different extensions).

How to do that?