You won't be able to do a simple serialization here because the class itself is not marked with the [Serializable] attribute.
However, after looking at the docs, it looks like the class is really just a helper for constructing and manipulating strings like "text/javascript". And based on the documentation of the ToString method, you can round trip a ContentType object merely using the ToString method and the constructor.
For Example:
ContentType ctype = ....;//your content type object
String serialized_form = ctype.ToString();
//save the string to whatever medium you like
...
ContentType ctype2 = new ContentType(serialized_form);
Debug.Assert(ctype.Equals(ctype2));
you can do whatever you want with your string above (write it to disk...whatever).