Flask TypeError:join（）参数必须是str或bytes，而不是'；文件存储'；

我正在编写一个Flask应用程序，用户在其中上载一个.stl文件，该文件立即附加到电子邮件并发送到域电子邮件。我正在努力获取附加文件，但不确定错误是什么。有人知道问题是什么吗

@views.route("/uploader", methods = ["GET", "POST"])
def uploader():
   if request.method == "POST":

       file = request.files["file"]
       msg = Message(body = "TEST", recipients = ["email@gmail.com"])
       file.save(os.path.join(app.config["UPLOAD_FOLDER"], str(file.filename)))
       with app.open_resource(file) as fp:
           msg.attach(file, "application/sla", fp.read())
       mail.send(msg)
       flash("File sent", category="success")

       return redirect(url_for("views.on_demand", user=current_user))
   else:
       return render_template("uploader.html", user=current_user)

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将file.save替换为文档中所述的“filename=secure_filename（file.filename）file.save（os.path.join（app.config['UPLOAD_FOLDER']，filename））”