Wrong ContentType of resized images when the mimetype has changed during the resizing

The image plugin is applying a new filename ending in .png when any original image is resized that is not a jpeg one.

This happen for .gif images with the default setup, while a gif image is still returned, and thanks to the attachment mimetype that is unchanged, is sent with a .png filename and a image/gif mimetype.

If you customize the image processor to support svg images, you have no other choice then returning png resized images (to follow the naming change and inability to decide another way), but the png returned are served with an image/svg+xml mimetype.

So, with this issue we are having two problem in opposition:

1. GIF image are renamed PNG and served properly thanks to image/gif mimetype
2. Image really resized into PNG  are unproperly served because their mimetype is kept

The best would be to delegate this work to the image processor, which is the one who knows best what it does.