上传伪技术~很多人都以为判断了后缀，判断了ContentType，判断了头文件就真的安全了。是吗？

/// <summary>
        /// 图片上传
        /// </summary>
        /// <param name="file"></param>
        /// <returns></returns>
        public JsonResult UploadA(HttpPostedFileBase file)
        {
            if (file == null) { return Json(new { status = false, msg = "图片提交失败" }); }
            if (file.ContentLength > 10485760) { return Json(new { status = false, msg = "文件10M以内" }); }
            string filterStr = ".gif,.jpg,.jpeg,.bmp,.png";
            string fileExt = Path.GetExtension(file.FileName).ToLower();
            if (!filterStr.Contains(fileExt)) { return Json(new { status = false, msg = "图片格式不对" }); }
 
            //todo: md5判断一下文件是否已经上传过,如果已经上传直接返回 return Json(new { status = true, msg = sqlPath });
 
            string path = string.Format("{0}/{1}", "/lotFiles", DateTime.Now.ToString("yyyy-MM-dd"));
            string fileName = string.Format("{0}{1}", Guid.NewGuid().ToString("N"), fileExt);
            string sqlPath = string.Format("{0}/{1}", path, fileName);
            string dirPath = Request.MapPath(path);
 
            if (!Directory.Exists(dirPath)) { Directory.CreateDirectory(dirPath); }
            try
            {
                //todo：缩略图
                file.SaveAs(Path.Combine(dirPath, fileName));
                //todo: 未来写存数据库的Code
            }
            catch { return Json(new { status = false, msg = "图片保存失败" }); }
            return Json(new { status = true, msg = sqlPath });
        }

/// <summary>
        /// 图片上传
        /// </summary>
        /// <param name="file"></param>
        /// <returns></returns>
        public JsonResult UploadB(HttpPostedFileBase file)
        {
            if (file == null) { return Json(new { status = false, msg = "图片提交失败" }); }
            if (file.ContentLength > 10485760) { return Json(new { status = false, msg = "文件10M以内" }); }
 
            //判断文件格式(MimeMapping)
            var contentType = file.ContentType;
            if (contentType == null) { return Json(new { status = false, msg = "图片提交失败" }); }
            contentType = contentType.ToLower();
            var extList = new Dictionary<string, string>() { { "image/gif", ".gif" }, { "image/jpeg", ".jpg" }, { "image/bmp", ".bmp" }, { "image/png", ".png" } };
            if (!extList.ContainsKey(contentType)) { return Json(new { status = false, msg = "图片格式不对" }); }
 
            //todo: md5判断一下文件是否已经上传过,如果已经上传直接返回 return Json(new { status = true, msg = sqlPath });
 
            string path = string.Format("{0}/{1}", "/lotFiles", DateTime.Now.ToString("yyyy-MM-dd"));
            string fileName = string.Format("{0}{1}", Guid.NewGuid().ToString("N"), extList[contentType]);
            string sqlPath = string.Format("{0}/{1}", path, fileName);
            string dirPath = Request.MapPath(path);
 
            if (!Directory.Exists(dirPath)) { Directory.CreateDirectory(dirPath); }
            try
            {
                //todo：缩略图
                file.SaveAs(Path.Combine(dirPath, fileName));
                //todo: 未来写存数据库的Code
            }
            catch { return Json(new { status = false, msg = "图片保存失败" }); }
            return Json(new { status = true, msg = sqlPath });
        }

/*头文件参考：（我自己测是如有偏差请联系我）
7790：exe，dll
5666：psd
6677：bmp
7173：gif
13780：png
255216：jpg,jpeg

8297：rar
55122：7z
8075：docx，xlsx，pptx，vsdx，mmap，xmind，“zip”
208207：doc，xls，ppt，mpp，vsd
 */
/// <summary>
/// 判断扩展名是否是指定类型---默认是判断图片格式，符合返回true
/// eg：file,"7173", "255216", "6677", "13780" //gif  //jpg  //bmp //png
/// </summary>
/// <param name="stream">文件流</param>
/// <param name="fileTypes">文件扩展名</param>
/// <returns></returns>
public static bool CheckingExt(this Stream stream, params string[] fileTypes)
{
    if (fileTypes.Length == 0) { fileTypes = new string[] { "7173", "255216", "6677", "13780" }; }
    bool result = false;
    string fileclass = "";
 
    #region 读取头两个字节
    using (stream)
    {
        using (var reader = new BinaryReader(stream))
        {
            byte[] buff = new byte[2];
            try
            {
                //读取每个文件的头两个字节
                reader.Read(buff, 0, 2);
                fileclass = buff[0].ToString() + buff[1].ToString();
            }
            catch (System.Exception ex) { return false; }
        }
    }
    #endregion
 
    #region 校验
    for (int i = 0; i < fileTypes.Length; i++)
    {
        if (fileclass == fileTypes[i])
        {
            result = true;
            break;
        }
    }
    #endregion
 
    return result;
}
/// <summary>
/// 图片上传（理论上需要二次渲染下图片，微软Save的时候有应该有一定的验证[我把含有一句话木马的图片上传，最后会返回一张空图片]）
/// </summary>
/// <returns></returns>
public JsonResult UploadC(HttpPostedFileBase file)
{
    if (file == null) { return Json(new { status = false, msg = "图片提交失败" }); }
    if (file.ContentLength > 10485760) { return Json(new { status = false, msg = "文件10M以内" }); }
    string filterStr = ".gif,.jpg,.jpeg,.bmp,.png";
    string fileExt = Path.GetExtension(file.FileName).ToLower();
    if (!filterStr.Contains(fileExt)) { return Json(new { status = false, msg = "图片格式不对" }); }
    //防止黑客恶意绕过，头文件判断文件后缀
    if (!file.InputStream.CheckingExt())
    {
        //todo：一次危险记录
        return Json(new { status = false, msg = "图片格式不对" });
    }
    //todo: md5判断一下文件是否已经上传过,如果已经上传直接返回 return Json(new { status = true, msg = sqlPath });
 
    string path = string.Format("{0}/{1}", "/lotFiles", DateTime.Now.ToString("yyyy-MM-dd"));
    string fileName = string.Format("{0}{1}", Guid.NewGuid().ToString("N"), fileExt);
    string sqlPath = string.Format("{0}/{1}", path, fileName);
    string dirPath = Request.MapPath(path);
 
    if (!Directory.Exists(dirPath)) { Directory.CreateDirectory(dirPath); }
    try
    {
        //todo：缩略图 +  水印
        file.SaveAs(Path.Combine(dirPath, fileName));
        //todo: 未来写存数据库的Code
    }
    catch { return Json(new { status = false, msg = "图片保存失败" }); }
    return Json(new { status = true, msg = sqlPath });
}