Use fixed encoding when opening non-Unicode files. It's useful, but we have code for adding a Detect from file entry directly in the combo box, as used in the file Open dialog. Using this there as well instead of having a separate checkbox strikes me as an improvement: simpler UI with less widgets and less code.
Well, IIUC the "use fixed encoding" is intended to save the user from having to switch the forced encoding on the open dialog from Unicode to fixed all the time, like on a Windows machine with a non-unicode default code page, but the user is also editing Unicode HTML. So if my understanding is correct its a useful convenience, not essential. But for sure moving it to the open dialog will make it more discoverable.