docs: function findParsableFile(book, filename) {
      var fs = book.getContentFS();
      var ext = path.extname(filename);
      var basename = path.basename(filename, ext);
      var basedir = path.dirname(filename);
  
      // Ordered list of extensions to test
      var exts = parsers.extensions;
  
      return Promise.some(exts, function(ext) {
          var filepath = basename + ext;
  
          return fs.findFile(basedir, filepath)
          .then(function(found) {
              if (!found || book.isContentFileIgnored(found)) {
                  return undefined;
              }
  
              return fs.statFile(found);
          });
      });
  }