In Moodle 3.3, the current list of files types supported for conversion can be seen on the Document Converter page by an admin user.
In the picture you can see that on one of my QA Testing sites, I have unoconv turned off and I am using the "new" converter plugin setup.
3.3 added the Google docs converter. Before this only unoconv was the only option and that needs to be set up on the server. If John is on a hosted site he may not have that option available. The site admin can set up Google docs conversion without changing the server configuration, but that only gives the three text document formats.