From Just Solve the File Format Problem
(Based on educated guesswork.)
The first three bytes are the ASCII signature "
The fourth byte is a version number: 0x10 for v1.0, or 0x20 for v2.0.
The fifth byte is a code for the original file format, and compression level:
0 = Windows BMPv3 1 = Windows BMPv3, max compression 2 = Windows BMPv2 3 = Windows BMPv2, max compression 4 = Sun Raster 5 = Sun Raster, max compression 6 = TGA 7 = TGA, max compression