为何AWS PutObjectCommand结合SST无需显式处理即可完成文件上传？

我正在试用SST工具，它能通过代码配置AWS服务与全栈Web应用。在其API教程中，我编写了如下文件上传函数：

import { Resource } from "sst";
import { getSignedUrl } from "@aws-sdk/s3-request-presigner";
import {
  S3Client,
  GetObjectCommand,
  PutObjectCommand,
  ListObjectsV2Command,
} from "@aws-sdk/client-s3";

const s3 = new S3Client({});

export async function upload() {
  const command = new PutObjectCommand({
    Key: crypto.randomUUID(),
    Bucket: Resource.MyBucket.name,
  });
  const body = await getSignedUrl(s3, command);

  return {
    statusCode: 200,
    body
  };
}

这个函数里完全没有显式处理文件的逻辑，我原本以为它只会在指定S3桶中创建一个空文件。但服务器启动后，我通过curl命令上传文件：

curl --upload-file somefile.txt "$(curl https://sometestapi.execute-api.eu-west-1.amazonaws.com)"

结果在AWS控制台查看S3桶时，却能看到somefile.txt及其完整内容。我搞不懂两个核心问题：

• a) API是如何获取到该文件的？函数里没有任何相关处理逻辑
• b) 文件又是如何传输至S3的？

打印command和body得到如下内容：

|  +26ms       {
|  +26ms         command: PutObjectCommand {
|  +26ms           middlewareStack: {
|  +26ms             add: [Function: add],
|  +26ms             addRelativeTo: [Function: addRelativeTo],
|  +26ms             clone: [Function: clone],
|  +26ms             use: [Function: use],
|  +26ms             remove: [Function: remove],
|  +26ms             removeByTag: [Function: removeByTag],
|  +26ms             concat: [Function: concat],
|  +26ms             applyToStack: [Function: cloneTo],
|  +26ms             identify: [Function: identify],
|  +26ms             identifyOnResolve: [Function: identifyOnResolve],
|  +26ms             resolve: [Function: resolve]
|  +26ms           },
|  +26ms           serialize: [AsyncFunction: se_PutObjectCommand],
|  +26ms           deserialize: [AsyncFunction: de_PutObjectCommand],
|  +26ms           input: {
|  +26ms             Key: '860d1393-4232-48fc-4d4b-b42c0c8a6473',
|  +26ms             Bucket: 'mytestprojectname-testbucket-tokfzzao'
|  +26ms           }
|  +26ms         }
|  +26ms       }
|  +26ms       {
|  +26ms         body: 'https://mytestprojectname-testbucket-tokfzzao.s3.eu-west-1.amazonaws.com/860d1393-4232-48fc-9d4a-b42c0c8a6473?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Credential=<removed-credentials>&X-Amz-Date=20240607T170451Z&X-Amz-Expires=900&X-Amz-Security-Token=<removed-security-token>&X-Amz-Signature=<removed-signature>&X-Amz-SignedHeaders=host&x-id=PutObject'
|  +26ms       }

我调试了函数中所有可用变量，未发现任何包含上传文件的内容。原本以为没有显式处理逻辑的话，文件不会被上传至S3。

补充一点背景：客户端实现相同功能的流程逻辑清晰，先获取预签名URL，再将文件上传至该URL，代码如下：

const image = await fetch(url // pre-signed URL, {
                    body: file // file from HTML form,
                    method: "PUT",
                    headers: {
                        "Content-Type": file.type,
                        "Content-Disposition": `attachment; filename="${file.name}"`,
                    },
                });

但上述服务器端示例中似乎完全缺失了最后一步，恳请解惑。

内容的提问来源于stack exchange，提问作者Josh Jahans