When adding a library type in the cms, you are presented with an error message if the type is not whitelisted in the web.config.
You edit the library file types and are presented with an error.
Not having the file extension whitelisted in the web.config.
Open the web.config and search for ek_LibFileTypeWhiteList.
Add the extension in to the string such as "*.iso".
Save the web.config and go back to the library properties and add the extension.