First a disclaimer... we make no guarantees that the thumbnail format will not change in future versions. Also, the following information was obtained by reading the code. It has not been tested to verify its accuracy.

The format of the thumbnail is:

%AI7_Thumbnail: width height bitsperpixel
%%BeginData: hexdigitcount Hex Bytes
color table data
image data

Both the color table and image data are ASCII Hex encoded. Each line of data is preceded by a '%' character to make them comments as far as
PostScript is concerned.

The color table is present only if bitsperpixel is 8 (I think this is the usual case but the code does appear to also support 24 bit data). The color table consists of 256 RGB triples. Each triple consists of three bytes in the order R, G, B.

The image data may or may not be RLE compressed. It is RLE compressed if the first three bytes are 'R', 'L', 'E'. Otherwise it is not RLE compressed.

The following algorithm decompresses the RLE data:

let b = next input byte
if (b != 0xFD)
    copy b to the output
else
    let b = next input byte
    if (b == 0xFD)
        copy b to the output
    else
        let n = b
        let b = next input byte
        copy b to the output n times
    endif
endif