How can I find out what values are available to the ContentType property? I know of "email", and "content", merely by guessing.
Workbench is not available for Marketing Cloud. A list of available content types for data folders can be found in the documentation.
Unfortunately it seem's, that not every listed ContentType is valid e.g when you try to create a automation folder using ContentType "automations" you will receive an error .... not a valid Folder ContentType
I think the best way to find this would be via the workbench.
To access the workbench, go to this URL: https://workbench.developerforce.com and login with your SF credentials.
Then click on the menu: Info > Metadata Types & Components
I think you might find the information you are looking for there.