Enum MimeType

// Use MimeType enum to log the name of every Google Doc in the user's Drive.
var docs = DriveApp.getFilesByType(MimeType.GOOGLE_DOCS);
while (docs.hasNext()) {
 var doc = docs.next();
 Logger.log(doc.getName())
}

// Use plain string to log the size of every PNG in the user's Drive.
var pngs = DriveApp.getFilesByType('image/png');
while (pngs.hasNext()) {
 var png = pngs.next();
 Logger.log(png.getSize());
}