title: WordPress function undefined: file_is_displayable_image()
body: <p>When I call the function file_is_displayable_image() from my template file, I get the following error:</p>
  
  <blockquote>
    <p>Fatal error: Call to undefined function file_is_displayable_image()</p>
  </blockquote>
  
  <p>The function works fine when I call using a hook, but I'm not sure why...</p>