parse_preset_input() uses input[len(_user_agent_args) + 1] to find
the FPS placeholder, but preset-http-jpeg-generic does not include
_user_agent_args at the start of its list (only preset-http-mjpeg-generic
does). The FPS placeholder '{}' is at index 1, not index 3.
This means the detect_fps value overwrites '-1' (the stream_loop
argument) instead of the '{}' FPS placeholder, so the preset always
uses the literal string '{}' as the framerate.
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