IMPROVEMENT browser ready image thumbnails.

Generate thumbnails in default format if original image isn't supported by browser.

For an instance tiff won't be displayed properly in browser, but it's an image and thumbnail can be generated by Imagick.

_config.php
```php
if(class_exists('Imagick')){
	Image::set_backend('ImagickBackend');
	File::set_class_for_file_extension(array('jpg','jpeg','png','gif','tif','tiff','pdf'),'Image');
}
```
This code example shows how to activate ImagickBackend and set additional extensions to be handled by Image class. All files except jpg,jpeg,png,gif will have thumbnails in png format.