下载nsurlsessiondownloadtask文件完成后检查内容类型

I am using a nsurlsession to manage downloading image/video files. Trouble is sometimes the request goes bad and returns text like "Content not found." How can I make sure that the file thats being downloaded is actually the file type I requested? (such as .jpeg or .mp4)I am using a nsurlsession to manage downloading