Hi Adam,
Sorry, but I don't understand your question. As I mentioned in previous post we use the latest (version 6.3.0.0) CuteEditor version. The FileProvider class is part of this dll. According to me, it's not possible (in a normal way) to use two versions of CuteEditor simultaneously.
We have created our own FileProvider based on the abstract class CuteEditor.Impl.FileStorage located in CuteEditor.dll version 6.3.0.0
Occording to me, the problem is server side. Can you confirm to me that, as soon as you click at a row in the table with the template files, your JavaScript (so Client Side) set the source of the IFRAME to the value of "url", the property of the row mentioned earlier?