renderer/Document: default to filename

Filenames will now be used if the DriveFile instance of the attachment
has no comment set.

packages/backend/src/remote/activitypub/renderer/document.ts

@@ -5,5 +5,5 @@ export default (file: DriveFile) => ({
 	type: 'Document',
 	mediaType: file.type,
 	url: DriveFiles.getPublicUrl(file),
-	name: file.comment,
+	name: file.comment || file.name,
 });