Upload files (images, pdf) with parameters to the

I would like to upload 1pdf file and array of images. I can upload 1pdf and 2jpg files, if jpg files more than two I get message:
Error Domain=NSCocoaErrorDomain Code=3840 "The operation couldn’t be completed. (Cocoa error 3840.)" (No value.) UserInfo=0x174268000 {NSDebugDescription=No value.}
Why is this happening? Also I tested server with POSTMAN and it work well.

NSURL *URL = [NSURL URLWithString:constFileUploadURL];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
[request setCachePolicy:NSURLRequestReloadIgnoringLocalCacheData];
[request setHTTPShouldHandleCookies:NO];
[request setTimeoutInterval:60];
[request setHTTPMethod:@"POST"];
NSString *boundary = @"0xLhTaLbOkNdArZ";

NSString *contentType = [NSString stringWithFormat:@"multipart/form-data; boundary=%@", boundary];
[request addValue:contentType forHTTPHeaderField:@"Content-Type"];

NSMutableData *body = [NSMutableData data];

[body appendData:[[NSString stringWithFormat:@"--%@\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
[body appendData:[[NSString stringWithFormat:@"Content-Disposition: form-data; name=\"user_id\"\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
NSString *userid = [NSString stringWithFormat:@"%li",userID];
[body appendData:[userid dataUsingEncoding:NSUTF8StringEncoding]];
[body appendData:[[NSString stringWithString:@"\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];


[body appendData:[[NSString stringWithFormat:@"--%@\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
[body appendData:[[NSString stringWithFormat:@"Content-Disposition: form-data; name=\"key\"\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
[body appendData:[constBackendKey dataUsingEncoding:NSUTF8StringEncoding]];
[body appendData:[[NSString stringWithString:@"\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];

for (NSData *data in arrayWithFiles)
{
    NSString *fileName;
    if ([arrayWithFiles indexOfObject:data] == 0)
    {
        [body appendData:[[NSString stringWithFormat:@"--%@\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
        fileName = @"Content-Disposition: form-data; name=\"files\"; filename=\"PdfFile.pdf\"\r\n";
        [body appendData:[fileName dataUsingEncoding:NSUTF8StringEncoding]];
        [body appendData:[[NSString stringWithString:@"Content-Type: application/octet-stream\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
        [body appendData:[NSData dataWithData:data]];
        [body appendData:[[NSString stringWithString:@"\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
    }
    else
    {
        [body appendData:[[NSString stringWithFormat:@"--%@\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
        [body appendData:[[NSString stringWithFormat:@"Content-Disposition: form-data; name=\"files%ld\"; filename=\"image%ld.jpg\"\r\n",[arrayWithFiles indexOfObject:data],[arrayWithFiles indexOfObject:data]] dataUsingEncoding:NSUTF8StringEncoding]];
        [body appendData:[[NSString stringWithString:@"Content-Type: application/octet-stream\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
        [body appendData:[NSData dataWithData:data]];
        [body appendData:[[NSString stringWithString:@"\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
    }
}

[body appendData:[[NSString stringWithFormat:@"--%@--\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];

[request setHTTPBody:body];
_connection = [NSURLConnection connectionWithRequest:request delegate:self];

Cod works, problem was on server side. File uploading 50mb, a post size 8mb.