form-data 里的文件可都是暂存文件系统的，你不主动调用不会全部读进内存。
但一个 JSON 字符串大概率你很难做流式解析（很复杂），严格解析完了可就全在内存里了。
参考：
The file contents are either stored in memory or temporarily on disk. In either case, the user is responsible for copying file contents to a session-level or persistent store as and if desired. The temporary storage will be cleared at the end of request processing.
