An example.
[attach=1] has a white background.
[attach=2] is the result after using the Magic Eraser in Photoshop.
The result isn't huge, but its visibly wrong. It would be easier to set the specific colour for transparent as the specific white of the background (The white of the fan, and white of the background, are slightly different, so Danmakufu, being the precise thing it is, should be able to distinguish between them, like it does with pure black (0,0,0) and almost black (1,1,1) ), than meet up with something like this.