title: JFile Chooser decide if Directory or File is selected
body: <p>My main goal:</p>
  
  <p>if the user selects a directory it scans the whole folder for mp3 files and returns them. If he selects some mp3 files it returns them.</p>
  
  <p>To return the selected files was an easy one but to scan the directory for mp3's isn't as easy as I first thought. 
  And I think to do that I first new to decide if the user selected a file or directory, but how? Since I can get both with <code>getSelectedFiles()</code>.</p>