先补点基础知识：https://imququ.com/post/four-ways-to-post-data-in-http.html

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boundary：边界数据是以multipart/form-data来编码以boundary开始内容描述回车字段具体内容如果是文件还要包含文件名和文件类型以--boundary--结束

问题描述

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解决方案：You shouldNEVERset that header yourself. We set the header properly with the boundary. If you set that header, we won't and your server won't know what boundary to expect (since it is added to the header). Remove your custom Content-Type header and you'll be fine，也就是取消自己设置Content-Type就好了