上传大文件发生内存泄漏

用例：
windows开server，命令为：
> .\gohttpserver.exe --upload

安卓机在局域网内，访问网页上传

问题：
每次开始上传大文件（800MB或2GB），进程占用内存就增加几十MB。
上传完成或取消后内存占用不降低。
多次上传后，内存占用可不断累加（目前最大值227.7MB），有可能导致崩溃

Reproduce the issue by starting gohttpserver.exe with --upload on Windows and uploading or cancelling an 800MB or 2GB file from the Android browser. Trace the upload handling and measure process memory before and after repeated uploads; done means memory no longer accumulates after uploads complete or are cancelled.

Written by the indexing model from the issue text.