Support for private UTIs in HLS CHARACTERISTICS tag

The HLS spec defines the characteristics tag as such:

The value is a quoted-string containing one or more Uniform Type
Identifiers [UTI] separated by comma (,) characters. This
attribute is OPTIONAL. Each UTI indicates an individual
characteristic of the Rendition.

A SUBTITLES Rendition MAY include the following characteristics:
"public.accessibility.transcribes-spoken-dialog",
"public.accessibility.describes-music-and-sound", and
"public.easy-to-read" (which indicates that the subtitles have
been edited for ease of reading).

An AUDIO Rendition MAY include the following characteristic:
"public.accessibility.describes-video".

The CHARACTERISTICS attribute MAY include private UTIs.

As far as I can see, ExoPlayer parses public.accessibility.transcribes-spoken-dialog, public.accessibility.describes-music-and-sound, public.easy-to-read and public.accessibility.describes-video, but it doesn't seem to be possible to retrieve other values (ie. private UTIs). So the last part from the HLS spec does not seem to be supported in ExoPlayer. Am I correct here, or am I looking in the wrong place?