function _readFile(fileName, callback) {
    fs.readFile(fileName, "utf8", function (err, data) {
        var content = "";
        if (!err) {
            content = data;
        }
        callback.apply(null, [fileName, content]);
    });
}
Extract content locally from the file system used fs.readFile()

@param {String} fileName - fullpath of the document
@param {Function} callback - callback handle to post the content back