jQuery File Upload plugin asks me to download the file, what is wrong?

I'm using https://github.com/blueimp/jQuery-File-Upload and I was able to upload and save the file to designated folder, and then I return the Json object. Then the browser (I use IE8) pops "File Download" dialog and asks me to download a file named "upload75bea5a4" with no extension. I just could not figure out what is wrong?

I am using the same plugin and it's working without any problems for me. I will post the codes I am using so that can help you. The C# code I saw at Scott Hanselman's blog (and I made few changes).

A class to store the properties of the file:

public class ViewDataUploadFilesResult
{
    public string Name { get; set; }
    public int Length { get; set; }
    public string Type { get; set; }
}

The upload code, which is called by the ajax:

    [HttpPost]
    public string UploadFiles()
    {
        var r = new List<ViewDataUploadFilesResult>();
        Core.Settings settings = new Core.Settings();
        foreach (string file in Request.Files)
        {
            HttpPostedFileBase hpf = Request.Files[file] as HttpPostedFileBase;
            if (hpf.ContentLength == 0)
                continue;
            string savedFileName = Path.Combine(settings.StorageLocation + "\\Files\\", Path.GetFileName(hpf.FileName));
            hpf.SaveAs(savedFileName);

            r.Add(new ViewDataUploadFilesResult()
            {
                Name = hpf.FileName,
                Length = hpf.ContentLength,
                Type = hpf.ContentType
            });
        }
        return "{\"name\":\"" + r[0].Name + "\",\"type\":\"" + r[0].Type + "\",\"size\":\"" + string.Format("{0} bytes", r[0].Length) + "\"}";
    }

The javascript piece which makes the magic:

$('#file_upload').fileUploadUI({
    uploadTable: $('#files'),
    downloadTable: $('#files'),
    buildUploadRow: function (files, index) {
        return $('<tr><td>' + files[index].name + '<\/td>' +
                '<td class="file_upload_progress"><div><\/div><\/td>' +
                '<td class="file_upload_cancel">' +
                '<button class="ui-state-default ui-corner-all" title="Cancel">' +
                '<span class="ui-icon ui-icon-cancel">Cancel<\/span>' +
                '<\/button><\/td><\/tr>');
    },
    buildDownloadRow: function (file) {
        return $('<tr><td>' + file.name + '<\/td><\/tr>');
    }
});

Take a look and make some tests.

I wrote an article about it.