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)
{F555195}
Regular image passed as argument (preview size was slightly reduced in 2nd patch revision)
{F555699}
Multiple files that fit in the notification
{F555197}
Too many files, overflow
{F555198}
Single file without thumbnail
{F555700}
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/