HSI JPEG is a proprietary raster graphics format associated with Image Alchemy, and other products from Handmade Software. It is related to JPEG, but has special features to better support image regions that have only a small number of colors.
Image Alchemy's documentation says that the file extension is .jpg, but most or all versions of the software actually use .hsi.
Files begin with ASCII "
An HSI JPEG file contains what looks like an embedded JPEG file, but which isn't quite in standard JPEG format.