Attaching files to comments in Pastel is simple. Just click the 📎 icon in the bottom right of a new comment box and choose which file you'd like to upload. 

You can attach multiple files to each comment.

Once you attach a file and post the comment, the file will be available in the detailed comment view in the sidebar. 

You can click on the attachment to preview and download it. You can also attach multiple files in replies to comments.

Those are the basics of file attachments. Check it out on one of your canvases here.