title: Check if Gdiplus::Bitmap::FromFile has returned a valid Bitmap
body: <p>I had assumed that Gdiplus::Bitmap::FromFile returns NULL if the file is not a valid image, but it returns a non-NULL even if I pass it a doc file, for example. There doesn't seem to be an IsValid method for Bitmap, or anything similar. </p>
  
  <p>So how do I know if  Gdiplus::Bitmap::FromFile has really loaded a valid image?</p>