WPF - Saving font to disk, then instantiating GlyphTypeface for font throws exception. Why?

Question

I am mucking about with WPF glyphs and documents and have run into a null reference exception in the .Net 4 framework.

I extract and save true-type fonts to disk as

Answer 1

According to XamlToys Doesn't work on framework 4.0???, the problem is in the extension of the file for partial fonts.

When I renamed the .ofttf files I save to .ttf, it all works again. Haven't got the foggiest idea of why that is though. Seems to be new in .NET 4.0.