string fullPath = @"D:\KwDownload\song\test.mp3";
string filename = System.IO.Path.GetFileName(fullPath);//文件名 “test.mp3”
string extension = System.IO.Path.GetExtension(fullPath);//扩展名 “.mp3”
string fileNameWithoutExtension = System.IO.Path.GetFileNameWithoutExtension(fullPath);// 没有扩展名的文件名 “test”