I believe to have a little of experience regarding the computer, and before modifying the changes via the Info windows, I did exactly the same you suggested or the info windows did.
I'm using the latest beta, so the 1.8.3.
However, as you can see, the default settings, that I actually just pressed, propose the extensions twice, and this may lead people into confusion.
https: // img.skitchNO LINKS ALLOWED/20110206-kq6fhb8e46eg16s157tgpx7npp.jpg
Infact, it looks like to get the editing functionality working properly, the element that must be changed is the one without the type specified (first field in the form).
So what's the point to have the same settings into two elements, if only one is working?
Assuming the element in the list which contains the mime type is the one that contains the settings used during the upload phase, why don't you remove the editing setting for that element?
Anyway, I just have had a second try. I created manually a file extension in the suffix manager as one (with a different extension) created by the info property. Even replicating the setting, the extension created manually doesn't edit the file automatically.
I believe there is some bug somewhere!