title: UIActivityViewController -- NSURL ActivityItemProvider default thumbnail
body: <p>I am trying to figure out what sets the default thumbnail under the paperclip (instead of the embossed Safari icon) when you use an NSURL to attach a link in a UIActivityViewController. I feel like I have seen websites such as google and the itunes store setting these thumbnails to be helpful images. I feel that the embossed safari icon is too vague and the user doesn't know what link he/she is sending.</p>
  
  <p>Is it perhaps something in the html headers for that link?</p>
  
  <p>Thanks!</p>
  
  <p><img src="https://i.stack.imgur.com/pFEIQ.png" alt="vague safari icon"></p>