Uploading multiple files with iOS SDK and deleting them locally when done

I have the following scenario: in my DocumentsDirectory there are user-created pdf-files. Now I want to upload them all to a webserver. When successfully done, i want to remove the file from the device.

My first problem is, that I do not know, when the upload is successful for a specific file. Additionally I get errors on some files.

At first some code:

-(void)uploadPDFs
{
    NSFileManager *fileManager = [NSFileManager defaultManager];

    NSString *docsDir = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];

    NSArray *contents = [fileManager contentsOfDirectoryAtPath:docsDir error:nil];

    for (NSString *filename in contents) {
        if([filename.pathExtension isEqualToString:@"pdf"])
        {
            NSURL *docsDirURL = [NSURL fileURLWithPath:[docsDir stringByAppendingPathComponent:filename]];

            NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
            [config setTimeoutIntervalForResource:10];
            NSURLSession *upLoadSession = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:nil];

            NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"http://%@/api/putPdf?name=%@&folder=", [defaults objectForKey:@"serverAdresse"], filename]];
            NSData *pdfData = [NSData dataWithContentsOfFile:docsDirURL.path];

            NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url];
            [request setValue:@"application/pdf" forHTTPHeaderField:@"Content-Type"];
            [request setHTTPMethod:@"POST"];
            [request setValue:[NSString stringWithFormat:@"%lu", (unsigned long)[pdfData length]] forHTTPHeaderField:@"Content-Length"];
            [request setTimeoutInterval:60];
            // UploadTask is a NSURLSessionUploadTask
            self.uploadTask = [upLoadSession uploadTaskWithRequest:request fromData:pdfData];

            [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];

            [self.uploadTask resume];
        }
    }
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
    dispatch_async(dispatch_get_main_queue(), ^{
        [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
    });

    if (!error) {
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"NO ERROR");
            // CAN I GET HERE THE UPLOADED FILENAME?
    });
    } else {
        NSLog(@"ERROR: %@", error);
    }
}

The error I get especially when there are many uploads:

(-[ViewController URLSession:task:didCompleteWithError:]) (ViewController.m:4025) ERROR: Error Domain=NSURLErrorDomain Code=-1001 "The request timed out." UserInfo=0x7fa3957028f0 {NSErrorFailingURLKey=http://192.168.100.150/api/putPdf?name=2015-03-26_AN_14.pdf&folder=, NSLocalizedDescription=The request timed out., NSErrorFailingURLStringKey=http://192.168.100.150/api/putPdf?name=2015-03-26_AN_14.pdf&folder=}