Dynamically loaded assemblies. Reflection & Dynamic Types

In my silverlight project when i user drops a file on the page, based on the file name I load another assembly dynamically to handle data it contains.
In the dynamic assembly I have a class e.g. book and a method which returns a List<book>, if the user drops another file type i may dynamically download an assembly that deals with cars; it will have the class car and a method which returns List<car>.
The point is that i don't want my business model project to know anything about the contents of these dynamic assemblies other than the fact that they have a 'GetData' method which will return a List<> of whatever items they deal with i.e. books, cars....

The code in my business model project is below. I need a way (I presume using reflection) to convert the returnObject from a plain old object to a List<> so that I can make use of it effectively in my business model project.        


public object BuildDataObject(bool Merge, string FileType, object Data, Assembly DownloadedAssembly, string _Project)
        {
            object[] _params = new object[3];
            _params[0] = Data;
            _params[1] = Merge;
            _params[2] = false;
           
            Type _Object = _Assembly.GetType(_Project + ".Get");
            MethodInfo mi = _Object.GetMethod("GetData");
            ConstructorInfo ci = _Object.GetConstructor(Type.EmptyTypes);
            object response = ci.Invoke(null);
            object returnObject = mi.Invoke(response, _params);
            return returnObject;
        }

//returnObject is a List<myCustomItem>

so using reflection i can find out what the originating Type of the object was like...
           
            Type originalType = returnObject.GetType();

...but I'm really struggling with how to cast 'returnObject' back to List<myCustomItems> so that I can use Linq on it.

Silverlight 4, C#4