--[boundary]\r\n

[headers]\r\n

\r\n

[content]\r\n

--[boundary]\r\n

[headers]\r\n

\r\n

[content]\r\n

[... ...]

--[boundary]--

multipart可以内嵌multipart，form-data类型包含mixed类型。

mutipart的每一个分块都由boundary开始，同时也由boundary结束

doundary = multipart:headers[boundary]

content-type与content-disposition的数据结构一致：

content-type = value;params

content-disposition = value;params

multipart/form-data

域名 = multipart:headers[content-disposition][form-data]:params[name]

文件名 = multipart:headers[content-disposition][form-data]:params[filename]

multipart/mixed

文件名 = multipart:headers[content-disposition][attachment]:params[filename]