C# 如何后台判断 file控件上传的文件类型

 info.ImgPathPart = this.hdnImgPathPart.Value;

                HttpPostedFile fileDoc = Request.Files[0];

                info.R_RecheckNum = Convert.ToInt32(this.lblR_RecheckNum.Text);

                string[] doc_type = ConfigurationManager.AppSettings["doc_type"].Split('/');

                if (!doc_type.Contains(Path.GetExtension(fileDoc.FileName.ToUpper())))

                {

                   

                    Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", "<script>alert('上传文件必须为JPG/PDF/RAR/TIF/VSD/ZIP/PNG格式(小于5M)！');</script>");

                    return;

                }