How to find the actual "Content_Type" of the NSURLRequest before downloading it

Question

I want to know the content_type of the NSURLRequest before it is downloaded.

As much as I searched I found the below code as solution :

NSURL *url = [NSURL URLWithString:urlString];
NSMutableURLRequest *httpRequest = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60];
[httpRequest setHTTPMethod:@"HEAD"];
[httpRequest addValue:@"identity" forHTTPHeaderField:@"Accept-Encoding"];
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *task = [session dataTaskWithRequest:httpRequest
                                                completionHandler:
                                      ^(NSData *data, NSURLResponse *response, NSError *error) {
    NSHTTPResponse *httpResponse = (NSHTTPResponse*)response;                                  
    NSDictionary *dictionary = [httpResponse allHeaderFields];
    NSLog([dictionary[@"Content_Type"] description]);
}];
[task resume];

But i am always getting the content_type as text/HTML, even if the URL points to an image. So it might be the content_type of the header which send because of HEAD set at setHTTPMethod:@"HEAD".

Please help me to find the actual Content_Type of the NSURLRequest before downloading it.