Previously, if a user put their mouse in just the right place (to the

right of a thumbnail), all thumbnails would fade. This was because the

condition for a thumbnail to not fade was for the mouse to be hovering

over the .file inside. This issue has been solved by selectively

fading out thumbnails and not fading out the entire #file-list

container.
This patch also replaces the use of a self = this binding in

JavaScript with _.bind(..., this).

Ran JS tests.
Manually verified that placing the mouse cursor near the thumbnails no

longer triggers the old behaviour of fading out all attachments.

Instead, the currently hovered over attachment does not fade out but

all other do.