Allow choosing *.smi as valid subtitle file

[BasixKOR]

Description of the feature or enhancement you'd like to see in HandBrake

I'd like to add my SAMI files as subtitles. It seems like FFmpeg supports this (Jellyfin uses them) and it is widely used in South Korea as a common file type for distributing subtitles.

What Operating System are you running?

Windows 11 21H2 22000.675

What version of HandBrake are you running?

Nightly 20220515151217-37c9b9f53-master

Where did you download HandBrake from?

https://github.com/HandBrake/HandBrake-snapshots

Activity Log, Crash Log or any other details

No response

[sr55]

You might try muxing them into the source file and see if they are picked up that way.