I am not sure if this would help but try to change the b variable to the bytes variable in the line below at least you would be able to view the file in a test editor
fs.writeFile("test.txt", b, "binary",function(err) { });
If you would like to have the numbers space separated try the code below:
var fs = require('fs');
var foo = "71%73%70%56%57%97%50%0%50%0%247%0%0%150%140%115%102%94%69%198%187%159%123%114%90%71%71%71%138%129%101%202%193%166%201%193%172%238%234%221%200%197%188%140"
var bytes = foo.split("%");
var b = new Buffer(bytes.length);
var c = "";
for (var i = 0;i < bytes.length;i++) {
b[i] = bytes[i];
c = c + " " + bytes[i]
}
fs.writeFile("test.txt", c, "binary",function(err) {
if(err) {
console.log(err);
} else {
console.log("The file was saved!");
}
});