Not all file formats get a thumbnail generated, but a default icon showing the file type or at least its extension. If by any chance a file that usually get a thumbnail doesn’t, please wait a while. Sometimes the thumbnail generation takes a bit longer than usual. If you think anything have gone wrong, please contact us and tell us the id of the file so we can investigate further.
Sometimes a thumbnail just can't be created automatically by QBank or you may want to change the existing one. You may do that from the detail view for a media. Just below the preview area to the right is a button where you can upload a new thumbnail and/or preview image for the media.