What it says in the title. I have a custom file type whose extension is hidden in Explorer, but it's visible in DOpus.
Thanks.
A feature, IMO. Extensions are quite important when managing files, else you can't see what things really are.
Explorer's propensity to hide extensions isn't something we subscribe to, outside the special case of .lnk on shortcuts. It's arguably a security issue as well, as it makes it easy for one type to masquerade as another.
What you can do in Opus is hide extensions and/or move them into a separate column in some or all folders or types of folders. If you're doing something like using a folder as a launcher for things stored in special file types, then that gives you a way to hide the extensions in that folder and make it look tidy, while still being able to see them everywhere else.
But there's no way to selectively hide a particular extension? In my case I've configured a file type which is a null file with a pin icon and a hidden extension, called "sticky note". I use it to leave notes in folders.
Not currently. It's something we could make more configurable in the future.
Ok. Please add it to your (no doubt long) list. 
Another reason I like to hide a select few extensions is that, where I can unambiguously identify the type from the icon, it removes clutter and eases renaming. Beyond those few cases, I'm definitely pro-extension - the hiding of extensions is one of the first things I turn off in Windows.