See the top of the Folder Formats FAQ:
I don't think anything has changed in this regard between Opus 9 and 10, except that the Content-Type drop-down (which I think is what you were setting to "Disabled") is no longer on the default toolbar. Turning off Content-Type detection in Preferences, as explained at the very top of the FAQ, will have the same effect as setting the drop-down to "Disabled". (Disabled is also the default in Opus 10, although that may not apply to you if you've imported a config from Opus 9.)