为什么这个字符串没有绑定到ASP.NET.CORE API操作中的文件名参数?

我正在用fiddler测试一个API,使用下面的header和body并在 http://localhost:50063/api/image :

User-Agent: Fiddler
Content-Type: application/json; charset=utf-8
Host: localhost:50063
Content-Length: 32330

{"filename": "bot.png", "file": "base64 image ellided for brevity"}

示例代码来自 tutorial

[ApiController]
[Produces("application/json")]
[Route("api/Image")]
public class ImageController : Controller
{

    // POST: api/image
    [HttpPost]
    public void Post(byte[] file, string filename)
    {
        string filePath = Path.Combine(_env.ContentRootPath, "wwwroot/images/upload", filename);
        if (System.IO.File.Exists(filePath)) return;
        System.IO.File.WriteAllBytes(filePath, file);
    }

    //...

}

首先,我得到了文件名为空的错误500。我添加了 [ApiController] 属性到控制器类,我得到错误 400文件名无效 .

当我在这里提出同样的要求时, filename 绑定到复杂类:

    [HttpPost("Profile")]
    public void SaveProfile(ProfileViewModel model)
    {
        string filePath = Path.Combine(_env.ContentRootPath, "wwwroot/images/upload", model.FileName);
        if (System.IO.File.Exists(model.FileName)) return;
        System.IO.File.WriteAllBytes(filePath, model.File);
    }

    public class ProfileViewModel
    {
        public byte[] File { get; set; }
        public string FileName { get; set; }
    }

为什么会这样?