To allow users to upload multiple files, add the File Upload element and enable its allowMultiple option.

To add a comment for every question, enable a question's showCommentArea option. Take note that not all questions support the comment area by default. For instance, Text and Long Text questions do not have the comment area available by default. You can programmatically enable the Comment area for Text and Comment questions as described at The showCommentArea property does not work for Text and Comment type questions. Alternatively, you can simply add another Comment text field to allow users to leave additional information about their response.