The issue was caused by mimetype returned by Openfire, which always returns text/html even though it is asked about the uploaded image!

To solve this issue I've added a workaround which checks file extension embedded in the received URL and if it is equal to one of the popular image extensions then preview will be downloaded.

Additionally, I've improved handling of SSL certificates served by HTTP File Upload capable servers (it is now possible to upload the file to HTTP server which has self-signed certificate after the user manually approves this upload).