Oct 03, 2017 · Video data: Most WebM files will contain VP8 or VP9 for video. For them we’ll transcode the video data to H.264. Some (rare) .webm files already contain H.264 video in which case we can simply copy the video data over and transcode just the audio. Audio data: A WebM file might contain sound encoded with Opus at 8-48kHz or Vorbis at 8-192kHz
In addition to text and HTML, any kind of data can be inserted as an alternative content of the main body - for example a word processing document with the same text as in the HTML field. It is the job of the e-mail client to select and show the best fitting alternative to the reader. Usually this field is used for calendar events and such.