Some ATI recording programs used their own proprietary file format. You might use some ATI tool to export it to a more common MPEG or other video format.
Details are far too slim in your post.
Also, Explorer does hide file extenstions if you don't set them to show. Why not change that setting?
Am operating XP Pro. Have captured TV programming on external drive using ATI, but cannot copy the file to DVD. Neither Nero, Studio 9, SonicMyDVD, nor DVD Shrink will accept the file. In Details view, the file Type indicated is not an extension, but rather the name of whichever player is used. I believe the file to be mp2, but am hamstrung because Nero rejects the file. Anyone?