STJ is a file format created by STMicroelectronics around 2004. It is a format to store video based around the STV0674 chip.
The file format is not designed to be played back, rather to store a series of JPEG'S and uncompressed 8bit audio at 8, 11.5 or 21khz.
The original STJ2MOV (Mac) and AVICREATE (Windows) software to decode is no longer around, though an open-source decoder is here. https://github.com/dun4d/demuxstj