I've just checked and this option has not been moved over to the new preferences,
I can deal with this in 1 of 3 ways,
1) Add the preference to the current movie preferences.
2) Have the nfo overwritten by MC if exists
3) Rename the .nfo to .info if exists.
I think many would object to option 2, I think it comes down to does MC really need a setting for this.
Would you have any objection if I just setup MC to rename the .nfo to .info without the option.
Other users may want to comment on this also.
For the time being, edit the 'renamenfofiles' tag in the 'config.xml' file, located in the MC settings folder, ('<renamenfofiles>true</renamenfofiles>')