以下是一个示例代码，演示了如何在ASP.NET Core中将文件上传的进度报告返回到前端。

首先，创建一个名为"UploadProgress.cs"的类，用于存储文件上传的进度信息。

public class UploadProgress
{
    public long BytesRead { get; set; }
    public long TotalBytes { get; set; }
    public bool IsFinished { get; set; }
    public decimal Percentage => TotalBytes <= 0 ? 0 : (decimal)BytesRead / TotalBytes * 100;
}

然后，在Controller中创建一个"UploadController.cs"，包含文件上传的Action以及进度报告的Action。

[Route("api/[controller]")]
[ApiController]
public class UploadController : ControllerBase
{
    private readonly IWebHostEnvironment _environment;

    public UploadController(IWebHostEnvironment environment)
    {
        _environment = environment;
    }

    [HttpPost("upload")]
    public async Task Upload(IFormFile file)
    {
        if (file == null || file.Length == 0)
        {
            return BadRequest("No file uploaded.");
        }

        // 文件上传逻辑
        var filePath = Path.Combine(_environment.WebRootPath, "uploads", file.FileName);
        using (var stream = new FileStream(filePath, FileMode.Create))
        {
            await file.CopyToAsync(stream);
        }

        return Ok();
    }

    [HttpGet("progress")]
    public IActionResult Progress()
    {
        // 获取上传进度
        var progress = new UploadProgress
        {
            BytesRead = // 获取已读取的字节数,
            TotalBytes = // 获取总字节数,
            IsFinished = // 判断上传是否完成
        };

        return Ok(progress);
    }
}

最后，在Startup.cs文件中注册上传控制器，并启用Multipart请求。

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();

    services.Configure(options =>
    {
        options.AllowSynchronousIO = true;
    });
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    // ...

    app.UseRouting();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
    });
}

通过上述代码，你可以在前端调用"/api/upload/upload"执行文件上传，然后通过"/api/upload/progress"获取文件上传的进度报告。