Add basic Attachment type definition

ts/types/Attachment.ts

@@ -0,0 +1,21 @@
+import is from '@sindresorhus/is';
+
+import { MIMEType } from './MIME';
+
+
+export interface Attachment {
+  fileName?: string;
+  contentType?: MIMEType;
+  size?: number;
+  data: ArrayBuffer;
+
+  // // Omit unused / deprecated keys:
+  // schemaVersion?: number;
+  // id?: string;
+  // width?: number;
+  // height?: number;
+  // thumbnail?: ArrayBuffer;
+  // key?: ArrayBuffer;
+  // digest?: ArrayBuffer;
+  // flags?: number;
+}