307 Internal Redirect(in Network tab of dev console) with
Non-Authoritative-Reason: Delegateas response header and you get
XMLHttpRequest cannot load http://uploads-slideshare.s3.amazons.com/. Response for preflight is invalid (redirect)error, means that your browser is redirecting an
httpsand it is failing. In my case it was because of the HTTPS Everywhere chrome extension. You want to uncheck Amazon Web Services from the extension for the upload to work. Also, if you have PrivacyBadger, then adjust it for the site if login fails.