First, there are some basics you need to understand about multipart/formdata posts. Each part consists of at least a NAME and a CONTENTS part. If the part is made for file upload, there are also a stored CONTENT-TYPE and a FILENAME. Below, we'll discuss what options you use to set these properties in the parts you want to add to your post.
首先,你需要理解一些关于multipart/formdata post的一些基础知识。每一个part包含至少一个NAME和一个CONTENT部分。如果这个part用来上传文件,那么也可以有CONTENT-TYPE和FILENAME这些信息。(from http://curl.haxx.se/libcurl/c/curl_formadd.html)