How do I force OS X to treat a file as a Folder? I happened to name some of my photo folders "*.Playground" (e.g., some date plus the ".Playground"), it got picked up happily by Xcode and now OS X treats it like a file associated with Xcode, even changing the filename doesn't help.

Does anyone know a way to force the file to be treated as a Folder?
I created a folder with two .jpg files in it, and renamed the folder "Today.Playground". As you say, it got associated with Xcode. (Nifty icon!)

So I changed the name to "Today.layground". At first, Finder wouldn't show me what was in it. (That is, I was viewing it in column view, and column to the right showed a preview of a generic folder, instead of listing what was in it.) I clicked on another item and then clicked on the folder again, and Finder properly showed the two .jpg files still inside.

It appears to me that that's all you have to do: look away and then look again.