ajaxFileUpload上传文件后提示下载的问题

在某些版本浏览器下ajaxFileUpload上传文件会提示下载，

1：为什么？

可以观察到，即便返回 JsonResult 在返回的头中也没有任何消息体，直接理解为文本了。

2：解决方案

前端：

function uploadImg(fimgi) {
    if ($("#fimg" + fimgi).val().length > 0) {
        //alert($("#fimg" + fimgi).val().length);

    }
    else {
        alert("请选择图片");
        return;
    }
    $.ajaxFileUpload({
        type: 'post',
        url: "/product/UploadProductImage?fimgi=" + fimgi,
        secureuri: false,
        fileElementId: 'fimg' + fimgi,
        dataType: "json",
        success: function (data) {
            alert("上传成功！");
            //alert(data.O);
            $("#Img" + fimgi).val(data.O);
        },
        error: function (XMLHttpRequest, textStatus, e) {
            alert(textStatus);
            alert(e);
        }
    });
}

后台改为范围ContentResult，且，ContentType = "text/html"。

public ContentResult UploadProductImage(int fimgi)
{
    HttpPostedFileBase head = this.Request.Files["fimg"+fimgi];

    if (head == null)
    {
        return new ContentResult
        {
            Content = new JavaScriptSerializer { MaxJsonLength = int.MaxValue }.Serialize(new R() { F = 1, M = "对不起，无上传内容！" }),
            ContentType = "text/html"
        };
    }

    var supportedTypes = new[] { "jpg", "jpeg", "png", "gif", "bmp" };
    var fileExt = System.IO.Path.GetExtension(head.FileName).Substring(1);
    if (!supportedTypes.Contains(fileExt))
    {
        return new ContentResult
        {
            Content = new JavaScriptSerializer { MaxJsonLength = int.MaxValue }.Serialize(new R() { F = 1, M = "对不起，只能上传 jpg, jpeg, png, gif, bmp！" }),
            ContentType = "text/html"
        };
    }

    if (head.ContentLength > 1024 * 1024)
    {
        return new ContentResult
        {
            Content = new JavaScriptSerializer { MaxJsonLength = int.MaxValue }.Serialize(new R() { F = 1, M = "对不起，大小超出限制1M！" }),
            ContentType = "text/html"
        };
    }

    var r = new Random();
    var filename = Guid.NewGuid().ToString("N") + "." + fileExt;
    string path = this.Server.MapPath("~/upload/product");

    if (!Directory.Exists(path))
    {
        Directory.CreateDirectory(path);
    }

    var filepath = Path.Combine(path, filename);
    head.SaveAs(filepath);
    string webPath = "/upload/product/" + filename;

    return new ContentResult
    {
        Content = new JavaScriptSerializer { MaxJsonLength = int.MaxValue }.Serialize(new R()
        {
            F = 0,
            M = "上传成功，保存为：" + webPath + "!",
            O = webPath
        }),
        ContentType = "text/html"
    };
}