ASP.NET核心图像文件验证错误。如何验证ImageFile？

public class UploadController : Controller
{
    private readonly IWebHostEnvironment _env;

    public UploadController(IWebHostEnvironment env)
    {
        _env = env;
    }

    [HttpPost]
    public async Task<IActionResult> UploadImage(IFormFile file)
    {
        if (file == null || file.Length == 0)
            return BadRequest("No file uploaded.");

        // 检查文件大小
        if (file.Length > 5 * 1024 * 1024) // 限制为5MB
        {
            return BadRequest("File size exceeds the limit.");
        }

        // 检查文件类型
        var allowedExtensions = new[] { ".png", ".jpeg", ".jpg", ".gif" };
        var fileExtension = Path.GetExtension(file.FileName).ToLower();
        if (!allowedExtensions.Contains(fileExtension))
        {
            return BadRequest("Invalid file type.");
        }

        // 尝试读取图像文件头以进一步验证
        using (var memoryStream = new MemoryStream())
        {
            await file.CopyToAsync(memoryStream);
            var image = Image.FromStream(memoryStream);
        }

        // 如果所有检查都通过，保存文件
        var uploads = Path.Combine(_env.WebRootPath, "uploads");
        if (!Directory.Exists(uploads))
        {
            Directory.CreateDirectory(uploads);
        }

        var filePath = Path.Combine(uploads, file.FileName);
        using (var fileStream = new FileStream(filePath, FileMode.Create))
        {
            await file.CopyToAsync(fileStream);
        }

        return Ok("File uploaded successfully.");
    }
}