Why can't I play all my videos on the PS3?
While the PS3 support a wide variety of Video formats it does not support them all check out the link below to see what formats are supported.
Since MKV files are an unsupported format they will be displayed with the title "Unsupported Data" like shown below, then if you try to play them you will get error message "The data type is not supported".
In other situations you might come across a video that looks like it will play indicated by the picture but when you go to play the video you get an error "The data type is not Supported. (800288C0)" This also means the video is encoded in a format that is not supported by the PS3 at this time.