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What is the format of video being played?
VideoPress converts your uploaded video file into MP4 and Ogg formats. Our MP4 files are encoded with H.264 video and AAC audio lossy compression codecs. Ogg files are encoded with Theora video and Vorbis audio lossy compression codecs.
What file types can I upload? ...
Video files are a packaging format containing multiple files (similar to a zipped file on your computer). VideoPress checks for known video packages before upload and attempts to decode your file’s audio and video compression formats once your file arrives on our servers.
VideoPress supports MP4 (.mp4, .m4v), QuickTime movies (.mov), Windows Media Video (.wmv), Audio Video Interleave (.avi), MPEG (.mpg), Ogg (.ogv), 3GPP (.3gp) and 3GPP2 (.3g2). http://en.support.wordpress.com/videopress/#additional-info