alwaysAvailableFile can't use mp4 files created by ffmpeg

[paperluigis]

Which version are you using?

1.18.2

Which operating system are you using?

Linux arm64 standard

Describe the issue

Describe how to replicate the issue

1. generate a noise.mp4 file by running ffmpeg -f u8 -ar 8000 -i /dev/random -t 4 -af volume=.125 -b:a 15k -vbr off -c:a libopus ./noise.mp4 (sample: https://github.com/user-attachments/assets/e66cdcdb-5b34-48b2-8741-b881c27f8f88)
2. start mediamtx, and observe the error

MediaMTX configuration

pathDefaults:
  alwaysAvailableFile: ./noise.mp4

MediaMTX logs

ERR: invalid 'alwaysAvailableFile': invalid stsz

Packet dump

No response