How to use addCustomHeaders for conversions?

[JakeCow]

How can I use addCustomHeaders to set conversions as public in a private bucket?
As described here: #868
I'm unable to find any documentation for it and there's nothing about it on Google either!

$user->addMedia($file)->
addCustomHeaders([
   'ACL' => 'public-read'
])->
toMediaCollection('videos', 's3');

The above makes media + conversions public.
Thanks!