When a notification contains URLs, they will be shown as a large preview (in case of one thumbnail) or multiple in a strip (similar to what Klipper does).

The previews can be clicked to open the file and can be dragged anywhere (e.g. to a webbrowser or chat window).

F555195: Screenshot_20161129_173221.png
Spectacle can take advantage of this (the cursor actually changes to the open hand cursor to indicate it's a draggable surface but this cannot be seen in the screenshot)
F555196: Screenshot_20161129_173311.png
Regular image passed as argument
F555197: Screenshot_20161129_173410.png
Multiple files that fit in the notification
F555198: Screenshot_20161129_173431.png
Too many files, overflow
F555199: Screenshot_20161129_173215.png
Single file without thumbnail - ideally it would then fall back to the small square representation but that is difficult to achieve

Clicking the thumbnails currently doesn't work because the MEL eats the click away. Also, right-click does nothing - would be neat to have the regular KIO file context menu there.
It also doesn't properly work with grouping, ie. when two notifications are grouped together, the thumbnail shown is the one from the last notification.

Needs https://git.reviewboard.kde.org/r/129586/