Hejazi's answer is simple and should work great.
If, for some reason, you need to be very specific for one request - for example, if you need to override headers, etc. - you can also consider building your own NSURLRequest.
Here's some (untested) sample code:
// Make a request...
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:myURL];
// Generate an NSData from your NSString (see below for link to more info)
NSData *postBody = [NSData base64DataFromString:yourBase64EncodedString];
// Add Content-Length header if your server needs it
unsigned long long postLength = postBody.length;
NSString *contentLength = [NSString stringWithFormat:@"%llu", postLength];
[request addValue:contentLength forHTTPHeaderField:@"Content-Length"];
// This should all look familiar...
[request setHTTPMethod:@"POST"];
[request setHTTPBody:postBody];
AFHTTPRequestOperation *operation = [client HTTPRequestOperationWithRequest:request success:success failure:failure];
[client enqueueHTTPRequestOperation:operation];
The NSData category method base64DataFromString
is available here.
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