Error Uploading MP4 Video Files after Update to 2.3

After updating the theme to 2.3, I could no longer upload any .MP4 files of any size using the Media Uploader. I would get the error: “Unexpected response from the server. The file may have been uploaded successfully. Check in the Media Library or reload the page.”

This has nothing to do with other plugins or server settings. After rolling back the theme to 2.2, everything worked as normal.

So something changed from 2.2 to 2.3 that has to do with the Media Uploader and MP4 files.

Thanks!