When I upgraded to WordPress 3.5, this plugin stopped working in my local environment. It still worked on my server, even though I have the exact same setup on them both. What's even more interesting is that it only failed the uploads when using AJAX.
So, after some investigation, I realized that the reason this is happening is because the
image_resize function - being used by this plugin - has been deprecated in favor of
WP_DEBUG turned on in my local setup, WordPress was emitting the following notice:
Notice: image_resize is deprecated since version 3.5! Use wp_get_image_editor() instead. in wp-includes/functions.php on line 2824
which originates from
Since the notice is being echoed right there in the response, the returned data is not valid JSON, so it seems like an error.