Dropbox says you can scrub through 1080p video files and get instant thumbnail previews, while audio files now display a full waveform preview when they're viewed on the Dropbox site. Dropbox also says that it now supports previews and comments "over 30" video and audio file types, so unless you're working with a truly esoteric format these features should work just fine. Support for time-based comments arrives today on the web and in the Dropbox iOS app, and it should be available on Android soon. Any Dropbox user can leave these comments, but only files shared by paid Dropbox users will have the feature enabled.