Facebook Share with File Plugin Show Image

I am trying to edit the default file plugin to show the image when a user shares the page. But i cant get it to show the image. Does anyone know how to do this?

Open graph for image file:

<meta property="og:image" content="http://MySite.com/mod/file/thumbnail.php?file_guid=275077&amp;size=master&amp;icontime=1418777470"&gt;

html on page for share button:

<a href="http://www.facebook.com/sharer/sharer.php?app_id=8995688945968&amp;sdk=joey&amp;u=http://MySite.com/list/?user=275077">Share</a&gt;