Make wp_prepare_attachment_for_js() compatible with plugins that disable all...

Make wp_prepare_attachment_for_js() compatible with plugins that disable all intermediate image sizes (so no images are created on upload), and then create them on the fly using the image_downsize filter (say, with a URL that can dynamically create images on the fly).

Props nacin
fixes #22598


git-svn-id: https://core.svn.wordpress.org/trunk@22850 1a063a9b-81f0-0310-95a4-ce76da25c4cd