Attachments: support for incrementalMac and chunkSize

ts/types/Attachment.ts

@@ -130,6 +130,7 @@ export type AddressableAttachmentType = Readonly<{
   path: string;
   localKey?: string;
   size?: number;
+  contentType: MIME.MIMEType;
 
   // In-memory data, for outgoing attachments that are not saved to disk.
   data?: Uint8Array;

ts/types/MIME.ts

@@ -48,3 +48,6 @@ export const isAudio = (value: string): value is MIMEType =>
   Boolean(value) && value.startsWith('audio/') && !value.endsWith('aiff');
 export const isLongMessage = (value: unknown): value is MIMEType =>
   value === LONG_MESSAGE;
+export const supportsIncrementalMac = (value: unknown): boolean => {
+  return value === VIDEO_MP4;
+};