How do I get a media file with IContentRepository(and is that the way I suppose to do it)?

I have uploaded an image to a folder and using it in a block. Then I tried to get the AlternativeText property of my ImageFile and it is null even though the file is published with set values. 

var contentRepository = ServiceLocator.Current.GetInstance<IContentRepository>();
var contentLink = new ContentReference(id);
var media = contentRepository.GetDefault<ImageFile>(contentLink);

All media propertys are null..