Kenneth,
I am not using GetTempFilePath in my custom provider. The code I showed you is from CuteEditor.dll I used .NET Reflector to investigate CuteSoft's source code and I discovered that CuteEditor has some code to load temprary files from disk. That's the source of this problem.
Let me say it again to make sure I am 100% clear: there is code in the CuteEditor DLL that assumes temporary files are saved on disk and therefore prevent us from using a custom upload provider. The problem is not in my provider, the problem is in CuteEditor.dll. To be even more specific, I can tell you the problematic line of code in CuteEditor.dll is the following:
The reason why this line of code is problematic is because it makes an assumption: it assumes that the temporary file is located on the server hard drive which is no true when we implement a custom upload provider. Therefore, this line of code fails with the error message I previously demonstrated.
Hopefully this is clear now.