title: How to change file_extension internally in Blender 2.65
body: <p>I am writing a python script for Blender and I am quite new. I want to change output file format in render option internally. For example, <code>context.scene.render.file_extension = "PNG"</code> . But it is a read-only variable so I get an error. Is there a way to change it in code? </p>
  
  <p>I am using Blender-2.65.</p>