Although after installing said extensions these file types open fine, have proper code coloring, etc., I’ve had the problem that they don’t open in Design view, which was a real pain!
As a workaround I found that if I did a Spell Check on these pages, Design view would open as a split screen and as long as I didn’t close the file or click the Code view button I was OK. But recently I found the real problem and a real solution!
It turns out that I was adding these file extensions to the Edit > Preferences… > File Types / Editors > Open in code view list prior to installing the Dreamweaver extensions for that file type. Which was obviously…well…opening them in the code view.