Opening a terminal in a folder that contains a flv file called "one" (one.flv) and running this command: ffmpeg -i one.flv one.mp3 the flv file is converted to mp3 with the same name and in the same location.

What is the command that can be added (as shown here) so that the same result could be achieved directly from PCManFM's "Open with" context menu?

By trial and error I have naively found something that works in a way - a command like

ffmpeg -i %f.mp3 

would convert the file to mp3 but put it in the home folder and with no name.
Using a such command creates a ".desktop" file in ~/.local/share/applications that can be edited (to change name and icon). I have added this command to context menu and edited the name and icon:

How to modify it so as to output into the same folder and with the same name?

Also: what would be the command that added in the same way would allow converting multiple selected files?