Hello mate, love the plugin.
I'm just experimenting on my own project using plupload on the frontend of a theme.
At the moment I'm using the default custom code demo, which I noticed is quite similar to your own. I've got it working so it uploads to the 'uploads' directory. However I was wondering how you integrated this with the WP media library?
Whenever I've tried to use wp_handle_upload() on the contents of the $_FILES['file'] I get a 500 error. I've also thought about just using wp_insert_attachment() once I've uploaded it but ideally I'd like to use wp_handle_upload() so it generates all the thumbnails etc.
I had the code working with a standard POST so not sure if it's anything to do with sending the file with AJAX?
Any thoughts would be much appreciated