The files that contain the .ddat file extension are temporary video files that have been created by the DivX Player. These files are created by the player as a video file is downloading from the Internet.
The .ddat files contain a partial AVI video file. The .ddat extension will be given to the file until the video download is complete. If the video download is interrupted, the download can resume at the point in which it left off according to the DDAT data. Once the download has been completed, the video file will be renamed with the proper .avi extension.
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