Add precondition for `writeAttachment`

js/modules/backup.js

@@ -20,6 +20,7 @@ const archiver = require('archiver');
 const rimraf = require('rimraf');
 const electronRemote = require('electron').remote;
 
+const Attachment = require('./types/attachment');
 const crypto = require('./crypto');
 
 
@@ -493,6 +494,10 @@ async function writeAttachment(attachment, options) {
     }
   }
 
+  if (!Attachment.hasData(attachment)) {
+    throw new TypeError('"attachment.data" is required');
+  }
+
   const encrypted = await crypto.encryptSymmetric(key, attachment.data);
 
   const writer = await createFileAndWriter(dir, filename);