Thanks for your question. I have good news for you. Here is the excerpt from the “Thumbnail Substitution Support, mla_viewer” section of the Settings/Media Library Assistant Documentation tab:
You can also assign a “Featured Image” to any Media Library item. For non-image items such as Microsoft Office documents the featured image will replace the MIME-type icon or document title in an [mla_gallery] display. Simply go to the Media/Edit Media screen, scroll down to the “Featured Image” meta box and select an image as you would for a post or page.
If you use link=file you should get the “bigger image which will open up a fancybox” behavior you want. If not, give me the specific lightbox plugin you are using and I will have a look.
I am marking this topic resolved but if you have any trouble assigning a Featured Image for your documents, post an update and I will investigate.
