Better preview for images with transparency or white images.

The current background of the file preview in the IMCE is white. When working with images that have transparency and/or are mainly white (e.g. a white round button), the image blends with the background and the content editor doesn't know how the image looks like in real.

Most image editors solve this issue by providing a white/grey dotted background to indicate transparency. I'd like to see the same functionality in IMCE. For most users working with jpgs / photos, there won't be a difference, but this would be really helpful for users working with transparent pngs.