fix conditional block style

- The logical operators must go at the end of the statement before the
  line break, instead of putting them in the beginning of the next line;
- The else statement must start in a new line;

packages/helpers/classes/attachment.js

@@ -79,17 +79,20 @@ class Attachment {
 
     if (contentType === 'undefined') {
       return;
-    } else if (contentType === 'object'
-      && content.hasOwnProperty('type')
-      && (content.type === 'Buffer')
+    }
+    else if (contentType === 'object' &&
+      content.hasOwnProperty('type') &&
+      (content.type === 'Buffer')
     ) {
       const buff = new Buffer(content.data);
       this.content = buff.toString('base64');
       return;
-    } else if (content instanceof Buffer) {
+    }
+    else if (content instanceof Buffer) {
       this.content = content.toString('base64');
       return;
-    } else if (contentType !== 'string') {
+    }
+    else if (contentType !== 'string') {
       throw new Error('String expected for `content`');
     }