You can specify the MIME Type of files that you wish to find in Google Drive and the Google Script, using the getFilesByType method, will list all the matching files.

function searchFiles() {
  var results = [];
  var types = [MimeType.GOOGLE_SHEETS, MimeType.GOOGLE_DOCS];
  for (var t in types) {
    var files = DriveApp.getFilesByType(types[t]);
    while (files.hasNext()) {
      var file = files.next();
      results.push(file);
      Logger.log([file.getDateCreated(), file.getName(), file.getUrl()].join());
    }
  }
}