I uploaded files but all I see is the default thumbnail and an hourglass icon.

ContentCube creates thumbnails in a system queue. Occasionally, when uploading multiple files, some of the thumbnails don’t show up right away.

The hourglass icon is just ContentCube’s way of letting you know that it is working on generating the thumbnail. Your files are already uploaded and in the system, and if you click on one you should see it in full preview mode in the preview window.