将图像从iOS应用程序上载到服务器时出错

我正在尝试从iOS应用程序将图像上传到服务器的简单任务。然而，运行后我收到以下错误消息：

错误：错误域=NSCocoaErrorDomain代码=3840"JSON文本没有以数组或对象开头，并且没有设置允许片段的选项。"UserInfo=｛NSDebugDescription=JSON文本没有用数组或对象和不设置允许片段选项开头。｝

知道为什么吗？以下是我分别针对客户端和服务器端的代码。

应用程序端代码：

-(void) uploadPhoto: (UIImage*) image{
    NSString *urlString = @"myServerDNS.com/upload.php";
    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
    manager.responseSerializer.acceptableContentTypes = [NSSet setWithObject:@"text/html"];
    NSData *imageData = UIImageJPEGRepresentation(image, 0.8);
    NSDictionary *parameters = @{@"foo":@"bar"};
    [manager POST:urlString
       parameters:parameters
constructingBodyWithBlock:^(id<AFMultipartFormData> formData)
     {[formData appendPartWithFormData:imageData name:@"file"];}
          success:^(AFHTTPRequestOperation *operation, id responseObject)
    {NSLog(@"Success: %@", responseObject);}
          failure:^(AFHTTPRequestOperation *operation, NSError *error)
    {NSLog(@"Error: %@", error);}
     ];
}

服务器端php代码：

<?php
$foo = $_POST["foo"];
$target_dir = "Folder/" . basename($_FILES["file"]["name"]);
if (move_uploaded_file($_FILES["file"]["tmp_name"], $target_dir)) 
{
echo json_encode([
"Message" => "The file ". basename( $_FILES["file"]["name"]). " has been uploaded.",
"Status" => "OK"
]);
} else {
echo json_encode([
"Message" => "Sorry, there was an error uploading your file.",
"Status" => "Error"
]);
}
?>