How to get files ending with particular extension

(From f in GetFiles("D:\Desktop\Document\Sections","*.*",SearchOption.AllDirectories)
Let xt = Path.GetExtension(f).toUpper.Replace(".","")
Where {"HTML","CSS"}.Contains(xt)
Select fp = f).toArray

Taken and adapted from 🦉 🔖 [CheatSheet] - Filesystem APIs