Not sure why (I moved the website to HTTPS a few weeks ago), but i had to disable the ":secure_url" in the plugin to make it work for attachments:
PHP Code:
// if(strtolower(substr($thisimg, 0, 5)) == 'https') //If https site // $secure_og = ':secure_url'; // else $secure_og = '';
With secure_url enabled:
Code:
Object at URL ... of type 'article' is invalid because a required property 'og:image:url' of type 'url' was not provided.
Test platform:
https://developers.facebook.com/tools/debug/og/object/