HTTPWebrequest上传文件--Upload files with HTTPWebrequest (multipart/form-data)

使用HTTPWebrequest上传文件遇到问题，可以参考Upload files with HTTPWebrequest (multipart/form-data)来解决

https://stackoverflow.com/questions/566462/upload-files-with-httpwebrequest-multipart-form-data?tdsourcetag=s_pctim_aiomsg