Get file extension in directory and open file

This is my case:
I have more than 10 directories inside one directory. Some of these directories contain a html or htm file.
I want to know if there is a way using c#. That I can check if there is a html or htm file inside the direcory.
By placing a default.aspx page inside of each directory to check for htm or html and open de file.
Adding a default path will not be a good idea becasue every day a new directory is created.
Any ideas?
Thank you.