Videos must be encoded to support Adobe Flash Player 9. Videos must be saved as either a .flv (Flash video), .mp4 (MPEG-4, H.264 encoding), .vp6 (True Motion), or .mov (Quicktime MPEG-4, H.264 encoding) file type.
There are a few noteworthy formats that are not supported: Quicktime (MOV) files that use the Apple Intermediate Codec (used in Final Cut Pro), DRM protected files from Microsoft (WMV) or the iTunes Store (M4V, MOV), WMV videos using the MSS2 video codec (for screencasting).
We advise you to always upload videos using: MP4 container format, H.264 video codec, AAC audio codec. MP4 with H.264/AAC is an ubiquitous format that most widely used editing, compositing, screencasting, and transcoding tools support it. If your tools do not, take a look at Handbrake, a free but powerful desktop transcoder.