get file extension with file name only

Question

Hii

I have an application in asp.net with c#.

In this application different files(like .txt, .pdf, .jpg, .doc)  are stored in one folder

Now i want to get extension from file name only

means if name of file is test.doc i want to get extension from asp.net page in which i have only name of file(ex test only, i dont have extension here)

Please help me

Thanks in advance
Rahul patel