Extract indesign preview

/*The Base 64 decoder is from Stephen Ostermiller's site:

http://ostermiller.org/calc/encode.html



Preview extractor from Thomas Nielsen

*/



file_name = '/d/thumb.jpg';



var END_OF_INPUT = -1;



var base64Chars = new Array(

   'A','B','C','D','E','F','G','H',

   'I','J','K','L','M','N','O','P',

   'Q','R','S','T','U','V','W','X',

   'Y','Z','a','b','c','d','e','f',

   'g','h','i','j','k','l','m','n',

   'o','p','q','r','s','t','u','v',

   'w','x','y','z','0','1','2','3',

   '4','5','6','7','8','9','+','/'

);



var reverseBase64Chars = new Array();

for (var i=0; i < base64Chars.length; i++){

   reverseBase64Chars[base64Chars] = i;

}



function decodeBase64(str){

   setBase64Str(str);

   var result = "";

   var inBuffer = new Array(4);

   var done = false;

   while (!done && (inBuffer[0] = readReverseBase64()) != END_OF_INPUT

      && (inBuffer[1] = readReverseBase64()) != END_OF_INPUT){

      inBuffer[2] = readReverseBase64();

      inBuffer[3] = readReverseBase64();

      result += ntos((((inBuffer[0] << 2) & 0xff)| inBuffer[1] >> 4));

      if (inBuffer[2] != END_OF_INPUT){

         result +=  ntos((((inBuffer[1] << 4) & 0xff)| inBuffer[2] >> 2));

         if (inBuffer[3] != END_OF_INPUT){

            result +=  ntos((((inBuffer[2] << 6)  & 0xff) | inBuffer[3]));

         } else {

            done = true;

         }

      } else {

         done = true;

      }

   }

   return result;

}



function setBase64Str(str){

   base64Str = str;

   base64Count = 0;

}



function readReverseBase64(){  

   if (!base64Str) return END_OF_INPUT;

   while (true){     

      if (base64Count >= base64Str.length) return END_OF_INPUT;

      var nextCharacter = base64Str.charAt(base64Count);

      base64Count++;

      if (reverseBase64Chars[nextCharacter]){

         return reverseBase64Chars[nextCharacter];

      }

      if (nextCharacter == 'A') return 0;

   }

   return END_OF_INPUT;

}



function ntos(n){

   n=n.toString(16);

   if (n.length == 1) n="0"+n;

   n="%"+n;

   return unescape(n);

}



var myString = app.activeDocument.metadataPreferences.getProperty("http://ns.adobe.com/xap/1.0/","Thumbnails/*[1]/xmpGImg:image");

myString = myString.replace("&#xA;", "\n");

jpeg_string = decodeBase64 (myString);

jpeg_file = new File (file_name);

jpeg_file.encoding = 'binary';

jpeg_file.open ('w');

jpeg_file.write (jpeg_string);

jpeg_file.close();