title: Get the file extension of the file chosen by OpenFileDialog
body: <p>I am using an <code>OpenFileDialog</code> to allow the user to select a file.  How do I then get the extension of the file that they chose?  I need to perform a different action depending on the type of file.  For instance, if they choose a PDF file, I need to launch a PDF viewer, but if it's am image, I need to show it in a <code>PictureBox</code>.</p>