Unknown stream found in container (likely a subfile): need help identifying/opening it There's a video I need to view, foreign language film. I was checking it with ffmpeg, and it seemed legitimate except the third stream was something I've never seen before. I want to know what it is and how to view the stream's contents.

I tried dumping it with

ffmpeg -dump_attachment:t:0 subs.txt -i input.mp4


Yet the file that was dumped is invisible...but showed up a number in the file folder. I'm using windows 8.1 and hidden folders/files are set to visible. Maybe my command is wrong and this is a subfile... instead of a information file..(i tried s:0 instead but no difference)? Or I can't dump this? Should I change from .txt to some other extension or what do I need to do to dump and view it?

I just don't know what it is, and i want to know what it is. If we assume it's a subfile, I should be able to open it in a text editor right? So how can I do that...preferably a safe way of doing so.
The 3rd stream in your input is a regular stream. Attachments are basically extradata attached to streams.

You would use

ffmpeg -i input.mp4 -map 0:2 -c copy -f data subs.txt