It states that the output is either JPEG or RGB bitmap. What is it in the case of a PPM thumb model? The output does not appear to be either a JPEG, bitmap, or PPM.

I've checked the header and it's certainly not PPM.
I've tried loading the buffer into a bitmap in Java and it fails.

I can however dump the PPM thumbnail via dcraw_thumb_writer, so it can be processed.



Could you please specify

Could you please specify camera model you use? In most cases thumbnails are JPEG, the RGB case is very rare.
It is possible that somewhat is broken, so I should test with your sample.

-- Alex Tutubalin @LibRaw LLC