[OC 7] Fix preview animation on uploading

When adding/uploading files, the preview is now animated.
When loading a list of files directly the preview is displayed directly.

Please review @owncloud/designers 

This is a regression from OC 6, but I think it's fine to have it in 7.0.1.
(discovered through an IE8 bug)

CC @karlitschek @craigpg