entity:file:undefined" plugin does not exist upload using curl to jsonapi
The usual troubleshooting step when it seems to work for others is to spin up a minimal D8 app and see if it occurs there. The minimal app worked as expected, which points to a module conflict. For me, the contrib module file_entity was causing this trouble. It took several efforts to get it uninstalled, and only by following the #8 comment on https://www.drupal.org/project/file_entity/issues/3060204 where I try to install it would I be able to finally uninstall it.