Thank you, now I found it for myself (The "?"-button on the side didn't show the information, but you're right, the button upside "? Help" has this information - I searched the web, sometimes the shortest way might be the best.)
Now it works.
For others, who might search for this question, here my solution so far:
/bin/sh -c "/usr/bin/ffmpeg -i \"%f\" -nostdin -c copy -f mp4 \"%f.mp4\" \&\& rm \"%f\""
This makes from a mkv-container (or any other container) a mp4-container, filename extended with mp4 - means file "movie.mkv" will now be "movie.mkv.mp4".
Its quick and dirty, but it is a real mp4-container produced by ffmpeg (mp4-container are not possible to make it from a live-stream because they need the metadata about the stream - how long etc. in the beginning of the stream, not at the end. So you need a postprocessing command to change a mkv in a mp4.