[request] Specify types allowed for Multipart body (#38190)

The request library allows anything to be put in a Multipart body as
long as it can be passed to `Buffer.from()`: https://github.com/request/request/blob/master/lib/multipart.js#L74

However the current implementation allows only `string` to be used as
the multipart body. In my use-case I needed to pass a `Buffer` instance
directly.

This change defines a type alias for `MultipartBody` that includes
`string`, `Buffer`, `ArrayBuffer` or `UInt8Array`.

types/request/index.d.ts

@@ -183,6 +183,8 @@ declare namespace request {
     type OptionsWithUrl = UrlOptions & CoreOptions;
     type Options = OptionsWithUri | OptionsWithUrl;
 
+    type MultipartBody = string | Buffer | ArrayBuffer | Uint8Array;
+
     type RequestCallback = (error: any, response: Response, body: any) => void;
 
     interface HttpArchiveRequest {
@@ -204,7 +206,7 @@ declare namespace request {
         chunked?: boolean;
         data?: Array<{
             'content-type'?: string,
-            body: string
+            body: MultipartBody
         }>;
     }
 

types/request/request-tests.ts

@@ -649,6 +649,44 @@ request(
 }
 );
 
+request(
+  {
+    method: 'PUT',
+    uri: 'http://mikeal.iriscouch.com/testjs/' + rand,
+    multipart: {
+      data: [
+        {
+          'content-type': 'application/json; charset=utf-8',
+          body: JSON.stringify({
+            _attachments: {
+              'dot.png': {
+                follows: true,
+                length: 269,
+                content_type: 'image/png',
+              },
+            },
+          }),
+        },
+        {
+          'content-type': 'image/png',
+          body: Buffer.from(
+            'iVBORw0KGgoAAAANSUhEUgAAAAIAAAACCAIAAAD91JpzAAAAA3NCSVQICAjb4U/gAAAAX3pUWHRSYXcgcHJvZmlsZSB0eXBlIEFQUDEAAAiZ40pPzUstykxWKCjKT8vMSeVSAANjEy4TSxNL' +
+              'o0QDAwMLAwgwNDAwNgSSRkC2OVQo0QAFmJibpQGhuVmymSmIzwUAT7oVaBst2IwAAAAWSURBVAiZY/z//z8DAwMTAwMDAwMDACQGAwGaMKL7AAAAAElFTkSuQmCC',
+          ),
+        },
+      ],
+    },
+  },
+  (error, response, body) => {
+    if (response.statusCode === 201) {
+      console.log('image saved as http://mikeal.iriscouch.com/testjs/' + rand);
+    } else {
+      console.log('error: ' + response.statusCode);
+      console.log(body);
+    }
+  },
+);
+
 request(
     { method: 'GET'
     , uri: 'http://www.google.com'