How to cancel a download

I have an app.net asp.net page where a user can upload some files. I am using jquery-file-upload . Based on some condition, I want to cancel the download from the server side, but it doesn't work. No matter what I do, the file always goes to the server before asp.net returns an error. For example, I can only save this on load:

public async Task<HttpResponseMessage> Post(int id, CancellationToken token)
{
    return Request.CreateErrorResponse(HttpStatusCode.InternalServerError, "Cant upload");
}

      

        
        
        
      

    

If I put a breakpoint on return

, I can see that it is removed as soon as the download starts, but I need to wait for the download to complete, and only then the javascript error handler is called. Is it possible to permanently complete the request by canceling the download?

Update 1:

I replaced jquery-file-upload with jquery-form and now I am using ajaxSubmit on my form. It didn't change anything. I also tried to implement DelegatingHandler

, for example:

public class TestErrorHandler : DelegatingHandler
{
    protected async override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {

        //throw new HttpException(403, "You can't upload");

        var response = request.CreateResponse(HttpStatusCode.Unauthorized);
        response.ReasonPhrase = "You can't upload";
        return Task.FromResult<HttpResponseMessage>(response).Result;            
    }
}

config.MessageHandlers.Add(new TestErrorHandler());

      

        
        
        
      

    

It didn't work either.

And I tried to disable the buffer on requests:

public class NoBufferPolicySelector : WebHostBufferPolicySelector
{
    public override bool UseBufferedInputStream(object hostContext)
    {
        return false;   
    }        
}

config.Services.Replace(typeof(IHostBufferPolicySelector), new NoBufferPolicySelector());

      

        
        
        
      

    

No play - it loads the whole file anyway before returning an error.

I only need to cancel the download request. Is this not possible with web api or am I missing something?