File › Open With asks for the parameters of an importer that declares
any, at their defaults, once the file is picked (dialog `pluginImport`),
and opens the file with the values set. `file.openWith`, `app.open` and
MCP's `open_file` take `params` and pass them to the importer;
`app.open` without them runs the importer on their defaults, with no
dialog. A file opened with Open With is added to Open Recent, as File ›
Open adds the files it opens.