从URL获取MimeType

• (NSString *)urlToMIME:(NSURL *)url {
  CFStringRef fileExtension = (__bridge CFStringRef)[url pathExtension];
  CFStringRef UTI = UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension, fileExtension, NULL);
  CFStringRef MIMEType = UTTypeCopyPreferredTagWithClass(UTI, kUTTagClassMIMEType);
  CFRelease(UTI);
  return (__bridge_transfer NSString *)MIMEType;
  }