I just wanted to open a thread for this myself. I guess the reason is that all the attachment are delivered like this in the source code:
Code:
attachment.php?attachmentid=12345&cid=99
Facebook will probably only look for things like
.jpg,
.png and so on.
But I would also be very interested in any solution for that. Using images for Facebook sharing certainly draws a lot more attention to a post.
And since my CMS articles are are visible to anyone I don't need any rights management for attachments here.