Hello Stan,
The class PostedFile is an internal class, this is why you cannot find it. And yes, UploadedFile is an abstract class and PostedFile inherits and implements UploadedFile methods. The UploadedFile class has InputFieldName property defined like this:
As you can see, it is defined as internal. Unfortunately, it cannot be implemented outside of the Telerik.Web.UI assembly.
All the best,
Genady Sergeev
the Telerik team
Consider using RadControls for ASP.NET AJAX (built on top of the ASP.NET AJAX framework) as a replacement for the Telerik ASP.NET Classic controls, See the product support lifecycle here.