Improve the bitmap and vector mime icons

* Vector files such as Illustrator files currently show the "image"
icon. I propose to add a new type to mimetypeIcon() called
"image/vector" and which would be used by SVG as well
* We can use the current image-svg+xml icon, but would simply rename it
to image-vector
* The black and white "image" icon looks too much like a settings icon.
A simple picture representation pasted over the file icon should work