/messagesendpoint regardless of message type, but the content of the JSON message body differs for each type of message (text, image, etc.). See the following documentation for information regarding the type of messages you want to send:
audio/mpegis an unsupported combination). Another reason could be that the video file doesn't have an audio track: then the error message contains something like
video/mp4/h264/NONEwhere NONE is an indicator that the audio track is missing.