aac

Two AAC files: a 1 second clip of 3.5 KB and a 5 second clip of 16 KB, both a 440 Hz tone at 24 kbps. This is raw AAC in an ADTS stream, with no MP4 wrapper, so it behaves differently from M4A even though the audio codec is the same. There is no metadata block, and some players cannot work out the duration without reading the whole file. That makes it a useful check for audio players, upload validation and tools that read length before playing. If an app accepts M4A but rejects these files, it is probably looking at the container and not the codec.