在ASP .NET CORE 5中上传大文件需要使用流式传输。以下是实现的步骤：

1. 在Startup.cs文件中添加服务配置：

services.AddControllersWithViews(options =>
{
  options.Filters.Add(typeof(MaxFileSizeFilter));
})
.AddNewtonsoftJson(options =>
{  
  options.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();  
  options.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;  
});
services.Configure(options =>
{
  options.MaxRequestBodySize = int.MaxValue;
});

2.在控制器中添加以下代码：

public async Task UploadLargeFile(IFormFile file)
{
  if (file == null || file.Length == 0)
  {
    return BadRequest("File is null or empty.");
  }

  var folderName = "Upload";
  var fileName = ContentDispositionHeaderValue.Parse(file.ContentDisposition).FileName.Trim('"');
  var fullPath = Path.Combine(folderName, fileName);
  var dbPath = fullPath;

  using (var stream = new FileStream(fullPath, FileMode.Create))
  {
    await file.CopyToAsync(stream);
  }

  return Ok(new { dbPath });
}

3.添加过滤器MaxFileSizeFilter：

public class MaxFileSizeFilter : IActionFilter
{
  private readonly int _maxFileSize;

  public MaxFileSizeFilter()
  {
    _maxFileSize = 4 * 1024 * 1024 * 1024; //4GB
  }

  public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
  {
    var httpContext = context.HttpContext;

    if (httpContext.Request.ContentLength > _maxFileSize)
    {
      var result = new ContentResult();
      result.StatusCode = (int)HttpStatusCode.RequestEntityTooLarge;
      result.Content = $"Maximum allowed file size is {_maxFileSize} bytes.";

      context.Result = result;
      return;
    }

    await next();
  }
}

通过以上步骤，即可实现ASP .NET CORE 5下上传大文件（超过4GB）的功能。