Use getimagesize if exif_imagetype does not exists
Use getimagesize if exif_imagetype does not exists

On some servers, the exif_imagetype is not accessible. Rely to getimagesize which is a little slower to access image type.
If these 2 methods are not allowed, just use the standard extension detection which is not really cool...

Signed-off-by: Stéphane HULARD <a24f38cb8c57ec2778c80a9f0c151933508927e5@chstudio.fr>

shulard/CDNThumbnailer