I hadn't used the HtmlControls.HtmlInputFile control for a while, and was getting
"Object reference not set to an instance of an object."
My form encryption type was correct, id name gibed, everything good. Since everything was “right” I thought it might do a W2K3 thing. (Wouldn't be the first time...) So a do a quick Existing Item copy to a W2K Server project and relaunch. Same error. Good! Not a W2K3 issue.
Reviewing the code again I saw that I didn't declare my HtmlInputFile control as protected!
So I need to remember: when complexed, do a .NET 101 code check.