title: How to validate image format in django ImageField
body: <p>Our project uses Python 2.7, PIL 1.1.7 and Django 1.5.1. There is an ImageField which works OK for many image formats, including bmp, gif, ico, pnm, psd, tif and pcx. However the requirement is to only allow png or jpg images. How can it be done?</p>
  
  <p>Upd. I know I can validate file extension and http Content-Type header. But neither method is reliable. What I'm asking is whether there's a way to check uploaded file content for being png/jpg.</p>