gistfile1.m

    STHTTPRequest* r = [STHTTPRequest requestWithURLString:POST_CONTRACT_URL];

    NSDictionary *dataToEncode = @{@"key0": @"object",
                                   // other data...
                                   @"key1": @"object"};

    NSData* jsonData = [NSJSONSerialization dataWithJSONObject:dataToEncode
                                                       options:NSJSONWritingPrettyPrinted
                                                         error:&writeError];

    NSString* jsonString = [[NSString alloc] initWithData:jsonData
                                                 encoding:NSUTF8StringEncoding];

    r.POSTDictionary = @{ @"data" : jsonString };
    r.encodePOSTDictionary = NO;

    [r setUsername:USERNAME
          password:PASSWORD];

    r.completionBlock = ^(NSDictionary *headers, NSString *body) {
        // Handle success
    };

    r.errorBlock = ^(NSError *error) {
        // Handle Error
    };

    NSString* filePath = @"path/to/file";
    [r addFileToUpload:filePath
         parameterName:@"pics_1"];

    for (int i = 0; i < [signatureFiles count]; i++) {
        NSString* filePath = @"path/to/file";
        UIImage* image = [[UIImage imageWithContentsOfFile:filePath] imageScaledToFitSize:CGSizeMake(320.0f, 240.f)];
        [r addDataToUpload:UIImageJPEGRepresentation(image, 0.7)
             parameterName:[NSString stringWithFormat:@"pics_%d", i + 2]
                  mimeType:@"application/octet-stream"
                  fileName:[NSString stringWithFormat:@"%@.jpg", [signatureFiles objectAtIndex:i]]];
    }

    [r startAsynchronous];