Dropbox is an online cloud storage service which allows you to backup photos, videos, files & documents to cloud storage and access them from anywhere. You can easily upload videos to your Dropbox account on iOS. Dropbox uses Apple's HTTP Live Streaming (HLS) protocol which is specially designed to stream video files on your iOS device. The quality of the stream is dependent on your wireless internet connection speed.
Following are the video file extentions that are supported by Dropbox app: .mov, .mp4, .m4v, .mkv, .avi, .wmv, .flv & .asf.
If you have a video file which is not support by Dropbox app then you may have to open it by using third party app that supports the specified video file format.
You can install PicBackMan's Dropbox uploader for Mac from the website and start backing up photos and videos.