Here you can find all information you should know when using Mellonn Speak.
The supported recording file extension are the following:
The maximum recording length supported is 2.5 hours. These are all very common file types for audio (except .mp4). It is not recommended to use either .mp3 or .mp4, because they aren't very efficient and can slow down the upload/download process. You can't watch the video, if you're using .mp4.