get file extension with file name only

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check msdn link

Hii

string extention = Path.GetFullExtension(Server.MapPath(Request.ApplicationPath + @"\Quatation\") + id);

i have stored files with id in my folder that can be of any type like .txt or .doc or .jpg etc..

but from my page i can retrieve only id and i want to get extension or full name with extension from my folder

means if the name of file stored in folder is 14.doc
i have id(14) in my page.

how can i get full name with extension or extension of file 14 from my page?

thanks in advance