All subtitle formats used to be enabled by default in older versions of
media3.
After the media3 library was upgraded to 1.3.0, Extractors should
specify a `SubtitleParser.Factory` explicitly. By default, no subtitle
formats are supported when using the now deprecated empty constructor of
Extractors.
Limit support to **WebVTT** and **TTML** which are the only true web
standards amongst all the subtitle formats supported by ExoPlayer.
Note that only subtitles embedded in MP4 and WebM files are supported
until Mastodon provides a way to upload subtitles separately.
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