Calibre is an open-source eBook management tool for converting and organizing digital books. It should be installed when using the Book plugin.
FFmpeg FileFlows Edition
Default FileFlows DockerMod for installing FFmpeg.
ImageMagick
ImageMagick, invoked from the command line as magick, is a free and open-source cross-platform software suite for displaying, creating, converting, modifying, and editing raster images.