为什么会出现无效的HTTP HEADER Content-Type？

<form action="https://oapi.dingtalk.com/message/send" method="post">
    agentid:<input type="text" name="agentid" value="49579608"><br><br>
    access_token:<input type="text" name="access_token" value="315155555f533977958e79c1174207f5"><br><br>
    touser:<input type="text" name="touser" value="manager3804"><br><br>
    msgtype:<input type="text" name="msgtype" value="text"><br><br>
    text:<input type="text" name="text" value="xiaomingdeqingjiadan"><br><br>
    <input type="submit" name="" value="通知">
    <input type="reset" name="" value="重置"><br><br>
</form>