It works, provided that I only do it on one file at a time. That is, I click one file to mark it, right-click it, select “Convert to JPEG” and watch it work.
However, if I select two or more files and then right-click one of them and pick “Convert to JPEG”, it attempts to send all the files’ paths at the same time to the same execute command, which of course just confuses FFMPEG, and it thus fails.
How do I make it so that it executes a dedicated command for each of the selected files? Neither search engines nor ChatGPT were able to tell me how.