Please note that the extension of the video file is not the only factor which determines whether the file can be played in the RadMediaPlayer. The .mp4 video file should be having H.264 as the video codec and AAC as the audio codec, then only it can be played in the RadMediaPlayer. Even if the file is having a .mp4 extension but if the codec are different like XVID or PCM then you cannot play such files. So if these requirements are not met, please convert the video file to the supported format with the prescribed codecs. You can use any video converting software which can convert video files to .mp4 format with H.264 video codec and AAC audio codec. You can try some of these converters available on the internet which are free to download and use.
Any Video Converter