So, today after upgrading to 3.5 I ran into the Media Uploader issue that a lot of people seem to be having. The many fixes i’ve found in the forums vary quite a bit and none of them worked for me. This was especially frustrating because the error doesn’t offer any insite into what is causing the problem — "An error occurred in the upload. Please try again later."
After a few hours of trouble shooting I was able to figure out what the issue was for me so I decided to post it here.
I had an attachment_link filter for images that was missing a variable.
add_filter( 'attachment_link', 'my_enhanced_image_link' );
I discovered the missing variable in a PHP Notice from an instance of the old media uploader which I had placed in a Custom Meta field. Sort of odd that a notice in a separate uploader would cause this issue, but alas…
Once I removed that filter (which proved to be unnecessary anyway) the new WP3.5 Media Uploader came back to life and has since worked correctly. Sort of disappointed in the UI of the new media uploader but that’s a topic for another thread.
Hopefully, this helps someone out.