C#邮件带附件发送成功后附件文件不能及时删除，报错该进程无法访问路径“”，因为它正在被另一个进程使用

背景
在做邮件发送因为附件file控件是动态产出的，在后台只能以httpfilecollection方式循环获取， 
因为IE和FireFox兼容问题，IE下File.FileName获取的是文件的全路径而在FireFox下就只能获取到文件名， 
导致Attachment attchment = new Attachment(File.FileName);火狐下会出错， 
因为File.SaveAs(File.FileName) 方法不存在FireFox和IE不兼容的问题， 
笨方法，先把附件通过File.SaveAs(File.FileName) 方法上传到服务器上， 
问题
邮件发送成功后再删除此文件，会一直报错异常“文件不能删除，另一个进程正在使用”；
原因及处理方式
以为File文件流的问题，真实原因是附件上传成功后需要释放的原因。 
代码如下：
string attchemUrlstrs = "";
//设置出错时提示的语句数组
string[] MsgStr = new string[3];
MsgStr[0] = "上传文件大小超过限制.";
MsgStr[1] = "上传文件扩展名是不允许的扩展名.";
MsgStr[2] = "上传文件失败\\n请重试.";
int MaxSize = 2048000;
string SavePath = "./../../Upload/Attachments/";
MailMessage msg = new MailMessage();
msg.From = new MailAddress(address);#
#region 循环添加附件
HttpFileCollection fileList = HttpContext.Current.Request.Files;
try
{
    int attSizeCount = 0;
    ///上载文件列表中的每一个文件
    for(int i = 0; i < fileList.Count; i++)
    { ///获取当前上载的文件
        HttpPostedFile File = fileList[i];
        //获得文件名
        string FileName = System.IO.Path.GetFileName(File.FileName);
        if(FileName != "")
        {#
            region 上传附件到服务器（ 这样做是为了ie和firefox不兼容， ie得到的是文件的全路径， 而火狐是文件名）
            string fileExtension = System.IO.Path.GetExtension(FileName).ToLower();
            string imgReName = "" + DateTime.Now.ToString("yyyy-MM-dd-HH-mm-ss-ffff", DateTimeFormatInfo.InvariantInfo) + "" + fileExtension;
            //文件夹名
            string imgFolderName = DateTime.Now.ToString("yyyyMMdd", DateTimeFormatInfo.InvariantInfo);
            try
            {
                if(!System.IO.Directory.Exists(@Server.MapPath("" + SavePath + "" + imgFolderName + "")))
                {
                    //生成文件完整目录
                    System.IO.Directory.CreateDirectory(@Server.MapPath("" + SavePath + "" + imgFolderName + ""));
                }
                File.SaveAs(@Server.MapPath("" + SavePath + "" + imgFolderName + "/") + imgReName);
            }
            catch
            {
                Response.Write("<script type=\"text/javascript\">alert(\"附件上传总大小不能超过200k\");</script>");
                return;
            }
            attchemUrlstrs += @Server.MapPath(SavePath + imgFolderName + "/" + imgReName) + ";"; //记录附件地址
            Attachment attchment = new Attachment(@Server.MapPath(SavePath + imgFolderName + "/" + imgReName)); //得到附件的绝对地址
            msg.Attachments.Add(attchment);
            attSizeCount += File.ContentLength;#
            endregion
        }
    }
    if(attSizeCount > MaxSize)
    {
        Response.Write("<script type=\"text/javascript\">alert(\"" + MsgStr[0] + ",附件上传总大小不能超过200k\");</script>");
        return;
    }
}
catch(Exception ex)
{ ///显示上载文件的操作失败消息
    Response.Write("<script type=\"text/javascript\">alert(\"" + ex.ToString() + ",附件上传总大小不能超过200k\");</script>");
    return;
}
#endregion
//邮件发送地址
string to = this.txtTo.Text;
if(to != "" && to != null)
{
    string[] ccAddress = to.Split(',');
    foreach(string emailTo in ccAddress)
    {
        if(emailTo.Trim() != "")
        {
            msg.To.Add(emailTo);
        }
    }
}
msg.Subject = this.txtSubject.Text; //邮件标题
msg.SubjectEncoding = System.Text.Encoding.UTF8; //邮件编码
msg.Body = this.content1.Text; //邮件内容
msg.BodyEncoding = System.Text.Encoding.UTF8;
msg.IsBodyHtml = true; //是否是html
SmtpClient client = new SmtpClient();
if(type == "0")
{
    address = address.Substring(0, address.Length - (address.Length - address.LastIndexOf('@')));
    client.Credentials = new System.Net.NetworkCredential(address, emailpass);
}
if(type == "1")
{
    client.Credentials = new System.Net.NetworkCredential(address, emailpass);
}
client.Host = server; //发送邮件服务器smtp：25，接收邮件服务器pop:110
client.EnableSsl = false;
client.Port = 25;
try
{
    client.Send(msg); //发送邮件 
    Response.Write("<script   language='javascript'>alert('邮件发送成功')</script>");
}
catch(System.Net.Mail.SmtpException ex)
{
    Response.Write("<script   language='javascript'>alert('邮件发送失败，原因是：" + ex.ToString() + ")</script>");
}
finally
{
    foreach(Attachment item in msg.Attachments)
    {
        item.Dispose(); //一定要释放该对象,否则无法删除附件
    }
    #region 不论邮件是否发送成功， 删除服务器上临时附件
    string[] attchemUrllist = attchemUrlstrs.Split(';');
    foreach(string attchemUrl in attchemUrllist)
    {
        if(attchemUrl != "")
        {
            if(System.IO.File.Exists(attchemUrl))
            {
                try
                {
                    System.IO.File.Delete(attchemUrl);
                }
                catch(Exception ex)
                {
                    ex.ToString();
                }
            }
        }
    }
    #endregion
}