An .rmid is a Standard MIDI File wrapped in a RIFF container: little-endian RIFF framing around a big-endian MIDI file. The SMF lives in a data chunk, so a MIDI can travel with RIFF-style metadata and tooling that speaks RIFF. Rare, but real. Hover any field or byte to light its span and read the decode; click a field with a + to open its table. Color marks kind (see the key).
The little-endian RIFF wrapper, then the big-endian SMF it carries. Two endiannesses in one file.
The wrapper, drawn above: magic, size, and the RMID form type.
A standard RIFF data chunk whose body is a byte-for-byte Standard MIDI File. Optional DISP / INFO chunks may sit alongside.
Inside the data chunk: a complete SMF. acidcat delegates it to the MIDI walker with offsets shifted into place, so the MThd/MTrk detail shows through at correct file positions.