[superagent] Improving the typing of the #field method (#20102)

There are two major changes here:

1) The value of a field no longer needs to be a string. It also
accepts arrays, buffers, blobs, streams, and booleans.

2) You may pass in an object, which sets all of the field values,
instead of calling once for each field.

types/superagent/index.d.ts

@@ -16,6 +16,10 @@ type CallbackHandler = (err: any, res: request.Response) => void;
 
 type Serializer = (obj: any) => string;
 
+type MultipartValueSingle = Blob | Buffer | fs.ReadStream | string | boolean | number;
+
+type MultipartValue = MultipartValueSingle | MultipartValueSingle[];
+
 declare const request: request.SuperAgentStatic;
 
 declare namespace request {
@@ -103,14 +107,15 @@ declare namespace request {
     interface Request extends Promise<Response> {
         abort(): void;
         accept(type: string): this;
-        attach(field: string, file: Blob | Buffer | fs.ReadStream | string, filename?: string): this;
+        attach(field: string, file: MultipartValueSingle, options?: string | { filename?: string; contentType?: string }): this;
         auth(user: string, name: string): this;
         buffer(val?: boolean): this;
         ca(cert: Buffer): this;
         cert(cert: Buffer | string): this;
         clearTimeout(): this;
         end(callback?: CallbackHandler): this;
-        field(name: string, val: string): this;
+        field(name: string, val: MultipartValue): this;
+        field(fields: { [fieldName: string]: MultipartValue }): this;
         get(field: string): string;
         key(cert: Buffer | string): this;
         ok(callback: (res: Response) => boolean): this;

types/superagent/superagent-tests.ts

@@ -285,6 +285,7 @@ request
     .attach('avatar', 'path/to/tobi.png', 'user.png')
     .attach('image', 'path/to/loki.png')
     .attach('file', 'path/to/jane.png')
+    .attach('fileWithOptions', 'path/to/file.png', { filename: 'filename', contentType: 'contentType' })
     .attach('blob', blob)
     .end(callback);
 
@@ -293,6 +294,12 @@ request
     .post('/upload')
     .field('user[name]', 'Tobi')
     .field('user[email]', 'tobi@learnboost.com')
+    .field({
+        field1: 'value1',
+        field2: Buffer.from([ 10, 20 ]),
+        field3: [ 'value1', 'value2' ],
+        field4: true,
+    })
     .attach('image', 'path/to/tobi.png')
     .end(callback);