Include the ability to add a link to online mockups (Like Invision, MarvelApp, Moqups, etc.) in addition to the current option of uploading regular image files.
Allow people to add links to Moqups, invision, balsamiq, etc. to the mockups section. Or even PDFs or BMML files.
Instead of trying to open them and treat them like we do the image-based mockups, we could just make a thumbnail with the logo on it (to show whether it's invision, etc.), and when clicked it links to or downloads the mockups. They would be shown inline with the rest of the mockups, though obviously missing the ability to comment straight on them. It's sort of a way of doing a poor-man's version of actually supporting external or non-image exported versions of mockups, while still adding the value of pulling mockups/designs all into one place.