MDL-32471 send_stored_file() now supports the preview option
If for any reason the preview image can't be generated for the given
file (eg we do not support its mimetype yet), the function will send
404 Not Found HTTP header. This will be useful for the lazy loading of
the file thumbnails as the JavaScript will simply ignore such response
and will not replace the default icon for the file.
In the future, a fallback thumbnail generator can be implemented that
would generate some sort of default preview for all files (eg using
the mimetype icon like some desktop OS do).
