I hadn't tried this feature recently, but I figured to give it a go.
After login in into Dropbox, there is a message that the browser is not supported.
I open the folder with a right click, and then tried to open the file for preview with a right click again, but nothing happens.
I wonder if it's a browser support issue. On the help video, it seemed to work fine: https://cafetran.freshdesk.com/support/solutions/articles/6000115264-source-document-preview-via-website
Igor, can you please have look?
It looks like Dropbox does not accept the Java internal web browser at the moment. It is probably because it lags behind with some latest browsing functions (e.g. the full support for html 5 standard). The rumors are that Java 9 web browser will catch up. Let's wait and see.
Thanks for the explanation, let's wait indeed!